We've checked the aerials (both indoor ones) but they've been fine for a while - the OnDigital box for over a year! Suddenly overnight both have a problem. Also, as i said, both RF power and RF Quality are at maximum levels.
Obviously not a problem from the transmitter then.
Well not resolved, but it's obviously not a transmitter problem. I'll have to fiddle with the aerials and see what i can do to get a better / stable signal. It's funny how things work fine for 2 years (in the case of the TV) without being touched and then overnight go wrong like this.