Thanks for that little piece of information, paulpaddison. I have the 4200DVB and it was set to RF channel 54, which clashes with Channel 4 in this area. I rang Telewest, got transferred around several times, and after 30 minutes being on the phone was told that they could not tell me how to change the RF frequency on the STB. However, they could send out a technician to do it, which would cost me £75.
I tried just the up arrow, but when that didn't work tried the up and down arrows, which got me into the diag screen. Changed the RF frequency, and everything is now ok. So that would have been £75 for 30 seconds work. Well done, Telewest. That's what I call customer care.