Hi, I have the same boiler with the same problem.
A fortune in repair bills later I found out it was a thermostat, unfortunately you need a 22mm spanner, grind the head width ways and bend the head to 90 degree's to get the thermostat off.
Or pay a foutune in repair bills.
P.S. this usually goes simultaneously to the diaphram.
Hope this helps,
Simon.
Friendly Simon.
March 2008