I remember in December 2015 when the BT exchange in York was affected by major flooding in the city, Minster FM and Radio York's kilostream links to the main FM transmitters were knocked out. Minster's web stream was unaffected though, so this was fed to the transmitter instead from a computer at Eagle Radio in Guildford. After a few days, the station's FM output ended up about 5 minutes behind DAB. I presume this must have been due to slightly slow processing, or maybe a bit of buffering, on the computer feeding out the web stream to the transmitter.
It's complicated to explain why this happens, but it's basically down to audio clocks on different devices not being perfectly in sync.
If for example you have a station encoding a stream with a 44.1kHz sample rate, their streaming computer will have a clock which is fired 44,100 times a second, which triggers the sound card to grab a sample of the incoming audio and pass it to the encoder. On the receiving end there's another clock running at the same rate, which is used to time when a sample should be received and output.
Unfortuantely as they're two separate devices they won't be in sync, and computers aren't perfect so neither will be precisely on time. This results in the output running slightly very slightly fast or slow (usually the latter), which results in the drifting on long running streams.
The neccessary buffering on the receiving end also doesn't help.
I’m not au fait with how local DAB muxes work (as you can tell). That’s very interesting...... Is there anywhere where I can read up on the setups?
I'm not sure there's a good public resource that explains it in simple terms, but if you look up Open Digital Radio they have some resources that go some way to explaining it.
Basically all DAB muxes have a multiplexer either at the transmitter or more often at a central location. They take in the incoming audio and data from each station, put it together in to the mux data stream, and then send it on to the DAB modulators at the transmitters create and send out the signals. There's a few options for getting stations to the multiplexer; stations can do all the encoding on site and send it to the mux ready to go, send audio to a encoder that the mux operator hosts, or simply tell the mux operator to use their web stream.
As for the specifics of what is used by each station and what's based where for the UK muxes, that'll be private knowledge between the mux operators and the individual stations.