Just as a thought Dave, PC interferance still plays a part in this theory... are you located where you can pick up another transmitter?
For example, where i live, i use the Winterhill transmitter however, every now and then, particularly the afternoon/early evening, my panasonic freeview equipment says there are new channels available. if i rescan, what my equipment acutally picks up is a welsh transmitter, and the signal is very, very weak. (weak enough to never get a good S4C picture on analogue) So my equipment replaces all the channel frequencies with the welsh ones, which gives a poor picture/signal drop, especially when traffic goes past or i use the microwave. Then in the morning, i get no channels at all as the signal drops that little bit in the mornings for some reason.
So is your freeview equipment looking for the BBC multiplexes on the right transmitter at the right frequencies?
BBC channels are usually the easiest ones to get on freeview with poor signals