Answer above is almost certainly right. I had exactly the same problem as you. If the air pressure in the expansion tank has dropped, but you increase the water pressure to compensate (by topping up with mains water), then the pressure will go over the pressure valve limit (causing it to vent) when the heat is on due to water expansion which should normally be taken up by compression of the air in the expansion vessel. Then when the boiler cools the pressure drops too low and the boiler won't then restart. Unless the expansion tank is actually leaking, you can re-pressurise it with air using a car tyre pump. There is a car-type pin valve at the top of the expansion tank at the top right of the boiler right at the back (accessible from the top of the boiler without taking any covers etc off, as long as the installer left enough room!). Start by bleeding water from the system until the pressure gauge reads zero. Then pump air into the expansion tank until pressure is about 1 bar. Then top up with mains water via the transfer connector until pressure is about 2 bar. This should fix the problem. The tank on my 601 drops pressure slowly but I've been pumping it up once a year for the last 5 years and not had a problem since.
Russ
November 2011
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