Ravenheat CSI 85AT fires up and goes of every 20 seconds?

I have a Ravenheat CSI 85at that is just over 3 years old. This is the one that does NOT have the honeywell gas valve and red ignition PCB or pilot light.
The problem is that the boiler on CH demand starts up then after about 5 seconds the fan turns off and the boiler closes down. Ten seconds later the boiler starts up and goes through the cycle again then closes down. This goes on and on and on. However if I run the hot water for 20 secs, once the hot water has been turned off the boiler works fine supplying heat to the CH system until it needs to fire up again due to timer swithing off or reaching thermostate level. Each morning I have to get up when the timer switches the boiler on to run the hot water for 20 secs so that the house will heat up.
After reading all of the comments on this excellent web site, I think the Green ignition PCB needs changing to resolve this problem? Does anybody know for sure so that I dont spend £70 unnecessarialy?
Thanks