Potterton Promax 15HE Boiler -relay clicking problem?

Hi, Need some help please.
A 6yr old Potterton Promax 15HE on an unvented boiler installation has suddenly stopped heating both DHW & CH. Had worked well without issues for 6yrs. Now when reset, fan starts up and runs while boiler does not fire up but has a relay on the ignition PCB clicking in & out for about half a dozen times and then locks system out. On checking found the following :-
-there is sparking on the electrode after reset;
-condensate trap was half full of water - drained this;
-gas valve (& hence gas feed) not receiving voltage, seems it is being inhibited by clicking relay action;
-resistance measurement cross various devices shows:- flow switch 11.63 kohms (not shorted so possibly ok); Thermistor 3.35kohms (within reqd range of 0.5k~20kohms so ok); safety thermostat 0.3ohms (seems ok); fan protection thermostat 0.3ohms (seems ok);sensing electrode high resistance to earth (not sure if this is ok).

Would greatly appreciate guidance on what to do next or advice on what may be wrong.
Many thanks.

km
January 2010