How can I fix my Potterton 16/22 Netaheat Electronic LPG boiler?

My boiler that, up to now, has given about 20yrs of excellent service, gives similar symptoms to the 'Prima F'. Fan OK plus pilot ignition OK. Ignitor then clicking every second up to two minutes plus before main burner cuts in, but only when starting from cold. When hot, only takes two/three seconds for main burner to cut in. Gentle tap (when cold) on the PCB triggers the main burner.
I have replaced both the electrode and the thermostat. Not overly expensive so far. Have read all the recent replies regarding the 'Prima F' with reference to replacing the electrolytic capacitors on the PCB 407677 and I am convinced that this may be the source of the problem (dry joints/weak capacitors).
However, my boiler PCB is a 407676 and appears slightly more complex than the 407677, (based on the web photos that I have seen of the 407677).
My question is, has anyone had similar problems and replaced the capacitors on the 407676 PCB?
If so, what capacitors did they replace, etc., etc?
Any help and info will be appreciated.