Valliant Gas (Butane) Water Heater won't fire up (yes- pilot on) ?

Heater won't fire up as the pressure 'through' the boiler is restricted/reduced. I've checked the water entry pressure which is fine - however the exit pressure is much reduced. I suspected that the problem lay in the regulation valve. I opened this up checked all the water ways were clear of obstruction etc.. but still to no avail. Any ides what the problem is ?