Graham,
On your guidance I entered the TV/Tuner section, item 2 on the service menu and was able to change from 16 to 3 - (what is that and what do the other numbers represent?) From there I went back into Menu, Tuning, TV and one of the items in the TV Tuning section is 'System'. I was able to toggle through, firstly to B/G without success, then to L at which point the system was able to search and find 6 terrestial analogue channels. (I will leave the challenge of moving to digital for another day).
The Avant is Type 81/03, SW (software???) 4.2. I purchased it nearly new from Carl Dyson in Edinburgh almost ten years ago and assume it is no more than 10 or eleven years old. Still does all the tricks and sits like a piece of sculpture in the living room.
Thanks again for your help,
Sigi