I checked out the Manchester SDR during daylight at that location (17:30) today. 1548 was Gold. Weak but steady. At the same time, The BBC flamethrower from the London area on 648 was missing, while 1458 was Gold on obvious local signal. To me, all of tht was sort of a mixed bag. 648 had a decent signal the other night, so that makes me think that the day signal isn't strong enough to break through the filter or whatever during daylight, despite the good nighttime signal. 1458 sounds exactly like it did the other night.
As for 1458, to my mind, the mystery continues. My best guess is that it was late emough in the afternoon that skywave may have been kicking in. The problem with that theory is that there was no fading....although daytime skywave is known for extended fade free periods. But it could have been a distant relay transmitter. Which, if true, might explain what sounds like convergence at night.
Indeed, as you alluded to, sometimes broadcasters in the UK and elsewhere in Europe, operate on a single transmitter with multiple transmitters. BBC Radio 4 on 720 with several low-powered transmitters is (or used to be) one example.
Whatever.... I'm looking forward seeing if I can unwrap this from the other side of the pond. As well as hearing other insights that anyone else here might have.
Fun fact: I haven't quite figured out which "Gold" transmitter is where, But,I DO know where the studio in London.. Unless, it's moved, it's on Leicester Square in the Theatre district of London. I've walked right by it,