5 minutes after reading Andy's first message, I rank Transco (9.00pm).
2 hours later one of their guys was round checking the gas pressure at the
meter - It turned out to be 12mb (should have been 20mb) so youre dead right-
there is a supply side problem.
1 hour later there were 3 guys digging up the supply pipe in the front garden.
(past midnight now).
When they cut through the supply pipe there was virtually no hiss coming out
at all.
They then stuck a flexible rod down the supply pipe towards the road.
It hit a blockage where it joins in the main supply that runs under the
pavement.
Now its 1:00am in morning - said they'd be back next morning to dig up the
pavement.
End of story.

Thanks very much for everyones help, advice and suggestions.
-I must remember to think of the system as a whole, and not concentrate on
most obviously affected part. And the real cause of a problem your
looking at can be miles away. (priceless lesson)

Thanks again - (hope someone finds this thread useful in future and saves
themselves a sunday taking the boiler burner to bits)

Steve