Thanks for your replies, chaps.
Yep, I'm in Somerset, 8009514.
I've had some success with my sister's Humax machine. Reverted to factory settings, as advised, then did a retune. Only trouble is bbc1 Wales is now channel 1 and bbc2 Wales is ch2. Channels 800 and 801 are the beeb1 and 2 that we should have. I can't work out to swap the channel numbers. I've tried everything. Arrrrrrgh.
Still can't get beeb 1 and 2 on my dad's set top box. There is no option in any of the menus to revert back to factory settings or shipping condition or anything like that. Haven't tried the aerial out option yet, mind you. I'll give it a go tomorrow and post back with the outcome.
Thanks again, chaps.