soot deposits on thermocouple?

Hi hope someone can help with this intermittent problem !!

On a Thorn Apollo 30/50 , we have recently had gas valve replaced.

However , after about a week the pilot light now leaves a soot deposit on the end of the thermocouple which I assume restricts the heat getting to it , and therefore shuts down the pilot flame. When the soot is cleaned off it relights and stays on , usually for about a week at a time.

I have removed the pilot tube & blown out any dirt & also renewed the pilot injector .

We have had Corgi tradesmen to date but its still not reliable.
Is there any other cause & solution of this problem I can try please?

Alan
January 2009