RAVENHEAT COMBI BOILER NOT FIRING NO HEATING OR HOT WATER?

I have a Ravenheat Combi CSl 85T Boiler it's 3 years old never been right since installation. Had two thermisters replaced over 3 years been serviced twice. The problem now is it will not fire up at all so I have no hot water or heating. Got plumber out who rang Manufacturers told him it was the valve.
It used to come on when I banged it several times but it doesn't even do that now. I am only hoping that it is the valve. On looking on the website it could be that it needs cleaning, as I have had the wall round by the boiler plastered but they covered the boiler up. Plumber got valve coming this afternoon to fit. He never mentioend about cleaning.
Does anyone have any other suggestions as I have a horrible feeling that I am paying for something and still boiler wont be right. Its cost me a fortune up to now -bring back the old cisterm. Fed Up to back teeth with it.

Thanks

Josie