Michael,
I'm not an engineer but, I've had similar problems.
The fault I found with my 28se was the heat exchanger was clogged solid with lime scale. try removing the heat exchanger, flushing through with kettle descaling fluid. it worked for me. Also check that the diaphragm hasnt become stuck, if so strip, clean, grease up (plumbers grease approx £3 for a tiny pot).
Good luck
Pete
pete
September 2005