I had this problem 15 months ago and noticed that the
mains supply voltage was at the bottom end of tolerance (221-216V) - when I added 6v to the mains
to the boiler (with a transformer) the problem cured.
Subsequently, after my complaint, the supply voltage
went to the upper end (245-252V). But now the
problem has recurred. I dare not add any more volts -
I guess that the valve seat/seal is getting stickly
with old age (28 years).
John Chambers
March 2009