Well I have just found the problem. A chap at work is an electrial
engeneer also knows a lot about plunbing previously worked as
maintence engeneer in a factory.He suggested the casing may not be
airtight. Or something to do with the air sensor not working
correctly. When the casing was off I noticed a small red plastic pipe
comming from a small unit in the top of the boiler to connect to the
inside of the neck of the flue, was twisted, I straigntened this and
low and behold when the casing was very carefull put back the boiler
light up. This small red pipe may have been moved when the flue was
being fitted. This little boiler gives plenty of hot water runs very
quietly and was £375 from B&Q. Thank to every one for the
suggestions..