Really interesting nations opts and network chatter.

They (the IBA) would have probably had two separate programme chains via their Colour Control Room (CCR) at
Emley. So two separate lines out of YTV to Emley, and a link/line onward from Emley to Belmont.

I've seen lines diagrams for Southern TV, that show similar arrangements to feed Rowridge and Dover (via the CCR at Chillerton Down). Yes, Southern TV Dover didn't/couldn't [1] directly feed the Dover Tx, their output went
via Northam (Southampton), because Northam played out both sets of East/West ads anyway

[1] Dover studios had local ends to GPO/BT Tolsford Hill, as did Dover Tx, so in an extreme case of Northam burning to the ground etc, Dover studio could have been patched directly by the GPO to the Tx

First point - there was not a main and spare distribution network.
There was a main and reserve feed of BBC 1 and BBC 2 from TVC to LO SWC only.
Crystal Palace usually sat on the main feed and the lines North South and West sat on the reserve.
Any chain could be switched to the other feed, or to another circuit via the matrix.
Crucially there was then only one line per network heading North South and West.
I can not overstate how expensive video circuits were.
I was on shift once around the 1 o'clock news time, and had a panic EMX call from a colleague in Manchester,
"I have lost both BBC 1 and 2 into me."
All I could do was feed BBC 1 into the Reverse Contribution circuit.
This was a video circuit from LO SWC to Birmingham, and then another line from Birmingham to Manchester.
That didn't work either.
It turned out that there had been a power failure in Pebble Mill Comms Centre.
They got BT (NSC or Anchor exchange in Birmingham) to patch the incoming network feed to them directly to the Manchester lines and take Pebble Mill out of the loop. Obviously the Midlands transmitters had to rely on RBR until the power was back, but everything north of that was OK.
I think you are confused that the Reverse Contribution (so named as most contribution feeds went into London, this one went from London) was a reserve feed. It was not, but it could be used if required, it just needed to be switched.

Second point, (yes LO SWC was London Switching Centre at Broadcasting House - here is the pulse and bar test signal https://www.flickr.com/photos/115059469@N03/12857014993, and here is a photo of a much younger and thinner engineer sat at the desk in there https://www.flickr.com/photos/115059469@N03/12857657534 - the network switching bit is just behind me).
All I can do is switch the outgoing feeds from me. I could not change what leaves Birmingham, Pebble Mill's Comms area did that.
They could route test signals to the feeds from them if for example we were sending material only as far as Birmingham on network distribution.

Third point - yes, it was only the feeds from the Comms Centres / CTA that went to the transmitters. No, LO SWC could only change the feeds leaving London.
I hope that helps!

During the GPO era, one vision circuit was charged at the cumulative rate of the number of truck phone circuits (aka revenue) it displaced, which was 960 (and no bulk discount). A national morning (08:00 to 13:00hrs) truck call back then was in the order of 15 to 20p a minute, so you were possibly dealing with a cost of 100 quid a minute for a vision circuit (at 1970/early 80s prices too !)

Network Pres would have been aware that there was going to be a feed on the distribution, but you are wrong to assume it was sent down the network circuits into BH. BH (LO SWC) switched the outgoing line to a different feed with the wanted material on.
It is possible that to make this look less obtrusive Pres took the music away with a fade beforehand. It was a hard switch of vision and sound so a fade would not be possible, nor anything else arty in the CTAs / Comms Centres / LO SWC.

As for Bristol's feed from Cardiff - this would have been possible using the Cardiff to Bristol contribution circuit that would have normally also been extended from Bristol to London, assuming it wasn't already in use for other things.

I presume you are filling in your overtime sheet for the week?

Ha! if only.....
The shift for the engineers was a fixed one, with little chance for overtime, unless a colleague went sick.
The way the bookings worked is we had a sheet for each quarter of an hour, showing what to switch when.
The bookings arrived from Circuit Allocation Unit (CAU) on the teleprinter to all the engineering areas affected by each switch, and you wrote them onto the correct page.
Somehow that day the printer got turned off in error, and after an hour we started to get questions as to why things hadn't been switched.
Then we realised how little had come off the printer in the last hour......

You've inadvertently answered a long standing question of mine, The Times archive tv listings refers to 'Transmitters Closedown' on BBC1 NI from time to time - typically when a Welsh language programme is airing in England. Did wonder whether the transmitters were literally powered down and then up half an hour later, which would have been fiddly to say the least. Although I seem to think that BBC2 transmitters at one stage in the seventies were literally off for several hours between Play School and the afternoon/evening programme session as a cost-cutting measure.

For many years there was a "closedown" at 1pm when Trade Transmissions stopped for lunch. Many transmitters took Welsh language programmes between 1pm and 1.30, when all transmitters then took a programme feed. Those stations that didn't take the Welsh programmes (Scotland, Northern Ireland and some other transmitters including Sandale) would continue test card to 1pm (the others went into a start of programmes sequence around 12.58 ) and then go to black and silence until they opted-in to the 1.30 programme.

Yes, that's right, from about 1978 until summer 1982. It was the imminent launch of C4 that
led to their reinstatement. The TV aerial trade were already annoyed by the lack of BBC 2 during most of
the day. Installing aerial and aerial systems properly requires the the presence of all available transmissions, to ensure all are being received correctly. Having only BBC 1 and ITV available did rather
hamper that. I don't know whether the IBA or the trade, or both lobbied the Beeb, but all day BBC 2 transmissions commenced again summer 1982, exactly when C4 test transmissions started. The BBC gave that as the reason for the re instatement in press releases.

(Of course Tennis and Cricket etc coverage still meant BBC 2 was alive on some days during summer)

I think this might help clear up the original question - 1985/6 vintage network routings

http://txlib.mb21.co.uk/main.php?g2_view=core.DownloadItem&g2_itemId=4001

So firstly, it seems that English regions didn't do opt outs on BBC2 until late 1986 so until then the circuits only went to the "big" centres with switching centres (eg Birmingham, Manchester, Bristol plus the nations) not the "smaller" centres (I think they are called Island sites in BBC parlance?)

Page 12 shows how the network got from Manchester to BT Carlisle, from where there were seperate BBC2 circuits to Belfast, Glasgow, Pontop Pike and Caldbeck.

At BT Carlisle, there were switching facilities under the control of BBC Manchester shown on page 32. On the BBC2 side, there was the main distribution circuit from Manchester and a "new circuit" which could be fed to various destinations.

So, to get a feed from London to Glasgow via the distribution circuit, it seems that the arrangement would be:

Manchester put test card on BBC2 distribution to all destinations including Carlisle
Feed incoming network into the "new circuit" to Carlisle
Make sure Glasgow has opted out
Switch New circuit onto Glasgow circuit at Carlisle

Therefore the Test Card G appearing in Northern Ireland would have originated in Manchester.

Some other interesting points from the documents - in that era some of the regional centres were only connected to the transmitters at opt out times, so the circuits were available for contributions. Others, like Leeds, could be bypassed by BT so the feed from Manchester could be sent directly to the transmitters, freeing up the circuits, so presumably if there was material being fed to Leeds via the distribution circuits, that's how they would have done it.

Not only that, but on BBC1 the test card appears to be subtitled - at least according to the ident that preceded it!

[beeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeep]