The 1969 fire: the transmitter at Black Hill had a slide scanner and a microphone, so the announcer - Drew Russell - was able to voice over some rudimentary captions between programmes. Black Hill had a network feed, so the Post office was able to switch programmes direct to the transmitter. They carried the 6pm edition of Granada Reports, which led with film of the STV fire which had been processed locally and then - if memory serves correctly - fed down the line from BBC Scotland's facilities at Queen Margaret Drive. It was a case of taking the scheduled programme if possible, and if not, whatever might be available from one of the two network lines.
I was at STV when a digger tore through the links from Cowcaddens to Black Hill, and on that occasion (as again when there was a power failure at Cowcaddens) Black Hill switched to an off-air feed of Grampian, rebroadcasting the Angus transmitter. There were receive aerials on the 1,000 foot mast which provided good quality rebroadcastable feeds of both Grampian and Border and latterly, Ulster as picked up at Darvel and fed by microwave to Salsburgh. When Grampian was rebroadcast, it included their commercials and continuity. I also recall the reverse happening at least once when a power failure knocked out Queen's Cross and Grampian viewers were switched to STV.
It is worth pointing out that until the advent of a common "ITV" programme, there was no actual "network feed" as the netrowk was reconfigured during the end break of every programme, and if you simply sat on that you might get crashes and splashes and perhaps not the programme you expected. Most stations had two incoming lines, one of which would normally be used for the fully networked programmes like Coronation Street. The other would carry feeds when regions took varying schedules - STV would take "Crossroads" from Southern for example as they shared a 1720 TX time, along with HTV. On very rare occasions, both network lines were busy and STV would take a particular programme from a third line that was fed a circuitous route through the Borders.
The BBC's UHF transmissions from Selkirk were simply a rebroadcast of Black Hill, so the caption would probably have originated at Queen Margaret Drive, as I don't think Black Hill had caption injection facilities for the BBC. In the days of 405 lines, all ITV main stations could radiate a Test Card that carried the legend "Reduced Power", and major BBC stations could do the same. However once UHF transmissions began, it was an expensive business to instal a dedicated colour slide scanner so there were fewer of those around. That was why the BBC (initially) switched to using the electronically-generated PM5544 card in the regions, as the generator for this was cheaper to instal and required almost no maintenance.
Fascinating stuff, as always, Tony.
I'm quite intrigued by the idea of transmitters being switched to broadcast an off-air feed from a neighbouring region, as I don't ever recall this happening in my neck of the woods back in the day. I remember breakdowns due to power failures, evacuations etc but all I remember seeing instead were captions from the transmitter, or on one occasion a U.S. comedy clean fed from a Granada line that was still open. The down time I refer to, however, rarely lasted more than an hour or so.
What factors would influence whether a neighbouring station should be rebroadcast? The anticipated down time, or the time of day, what programmes were being missed, etc? And who was responsible for making that decision?