soot deposit on thermocouple

[digital-des]

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 happlily relights and stays on , usually for about a week / fortnight at a time until it needs cleaning off again.

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

I think I need to know what causes the soot to build up on end of thermocouple , to arrive at the solution of this intermittent problem .
Any other solutions ./ ideas welcome !
Thanx
Alan