thelia 23e saunier duval?

I have a D.H.W. problem that i thought i had fixed by replacing the diapdragm. All was well for a week, now the main burner will not kick in, even though the boiler ignites as soon as a tap is opened.
It will fire sometimes and produce really hot water, but then for no apparent reason it will not. And so on,
The boiler was brand new when fitted in 2009 even though it was an old model, it had been stored boxed as new.
Occasionally it will work for d.h.w. in the summer position, but all very hit and miss.
Any help really appreciated....