Baxi Boiler Only Firing up sometimes?

Hi all,

Wonder if someone can help me ASAP. I have a old Baxi Boiler FS 601 which has a tank in the bedroom. Now I used this same problem 2 years ago and I replaced the valve and everything worked fine since then until now.

Problem I have is that the boiler SOMETIMES does NOT fire up when the programmer is set to kick off the central heating and water. Sometimes it works just fine, but sometimes it does not. If it has not fired up, I then override teh timer manually and it works just fine, the boiler fires up straight away. On the occasion when it fails to fire, the valve moves so it is not like the valve is jammed. The boiler clicks and begins to make a kettle sound but the pilot light just remains low. I am wondering what the issue is, surely the valve cannot have devloped a fault again as I replaced it 2 years ago? Just to add...when I first had the problem 2 years ago, I thought the programmer had gone but it was not.

Any advice would be really appreciated. I am afraid of calling a plumber and them not knowing what to look at.