Now this is kinda weird. When I was up the ladder moving around the aerial for digital television, I was trying to tune in the UHF frequencies of the Kings Cross transmitter. This is sensible because I only bought a UHF aerial, whereas Gore Hill transmits on VHF.
Tonight I got home and thought I'd do a bit of playing around with czap and the like. Scanning with that I found I could tune in, using mplayer, all the digital channels. Weird! So I tried scanning with MythTV again, to get the same results I had on Wednesday. When I imported czap's output into MythTV, however, I found I could tune the full complement of channels!
So looking at the frequencies, my humble UHF antenna is actually tuning in the VHF transmissions from Gore Hill! I guess the only explanation could be that the huge power they're outputting is enough to reach here.
That's excellent news as I was starting to despair of getting digital TV at all. Now to run the rest of the cable, drill the holes in the floor and get the Myth box going!