have you determined which cylinder is not firing ?
Sids water test
if yes have you tried swapping plugs over to see if fault transfers to other cylinder ?
if fault transfers then plugs are the problem if it doesn't then still on the water route if cylinder faulty is the front.
did it have the same petrol in it for 11 months ? if so how full was the tank (condensation builds up on the inside of half empty tanks) and after 11 months the petrol was probably shot anyway.
howevr i digress because your problem seems to manifest on wet days i still think there is water in the electrical sysstem somewhere, but do try the plug swap