OK, last part of the saga!
It looks as if there were several things that went wrong together.
Thanks to vag80 and mikes2 for their help comments and particularly vag80 who arrived at my door and went about three hours later having largely sorted it it out.
There was an air lock -- not sure where but it was BIG.
Bit disconcerting to see vag80 pulling off the various cooling heater hoses with the motor running. In fact there was little ( virtually no ) water from several of them. He reckons the pump is also weak.
Anyway once the water was flowing reasonably from all hoses..... on to the next bit.
The dash light was then flashing ( low water /overheating).
vag80 suggested the sender could be the culprit:
the sender had broken.... at the same time.... it was oK when I started but not afterwards.... honestly!
Ordered a new one from the Dealer ( seems to be an Audi only part)
Part came in , looked very different, but went with it, cut off the old connector they said and here are two new spade connectors to put on....
Light stopped flashing BUT the guage didn't work
Mikes2 checked out part numbers and sent me a link to an obsolete parts organisation and a VW dealer in Kirkaldy happened to have the original sender in stock and at half the price Audi wanted.
Big THANKS to Mike.....
http://www.partspool.com
car is now working properly, as far as cooling is concerned and I'm now moving on to changing the power steering fluid and arb bushes....