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.