Potterton Puma 80 c/h intermittently working until warmed up?

My boiler, Potterton Puma 80 (may be an 80e as no pilot light?) has been working fine for 5 years and now it takes about an hour before the central heating starts to work when cold (first thing in the morning and early evening). It fires up for about 3 mins and then shutsdown, then fires up again for a few seconds, like topping up. It does for about an hour then it stays on. It is like something is warming up. Hot water is all fine works ok, pressure is 1.5bar, no leaks etc. If heating is left on 24x7 works perfect!
Any ideas where to start would be helpful, I have already swapped the thermister, just in case, now looking at PCB.

Thanks