Potterton Prima 30B

Has anyone got any experience of the following problem with an 8 yr old Potterton Prima 30B?
The boiler will intermittently cut out after varying periods of time. It can go for about a week with no problems, and then cut out several times a day. It can then behave itself for a few days and then cut out again. There is no pattern to this.
I have been with the boiler when it cuts out and there is a definite "click" as it shuts off. It appears to come from the front right hand side, where the boiler thermostat is located. Someone has mentioned that this may be a safety cutout, but due to what?
I have had two heating engineers visit, one looked at the boiler from the other side of the room and said it needed replacing and the other did actually look at it and said it needed a part (not specified but cheap enough) and would be back. I have never been able to contact him again.
Before I get a third engineer in I thought it would be good to get an opinion from this forum - thanks

Hi Danielle. I do know someone with a 60B who has the same problem as yourself but it is not as often as yours. The longest it has gone this time is 2 months.
You could try increasing the pump speed if not on max already and opening up the bypass valve (if fitted) a touch. Just do one adjustment at a time to see which has hopefully cured your problem.

Hi Both

I had the same on my Potterton Prima 30B, there is a deflector tube that bends the pilot light flame onto the thermocouple, the tube gets sooted up and the flame hardly reaches the thermocouple.
I just de-coked the small tube with a pipe cleaner bent over on itself.

ATB Len