could be a few reasons why the pilot has gone out. possibly overheated, thermocouple worn, slow fan stopped?
to check these you will need to get inside the combustion cover but need to be corgi to do this, looks likely to be a call out but normally easily fixed. British Gas seem to have a lot of one off charge offers may be worth a shout.
when trying to light once flame is established ensyre you are holding the button in for minimum twenty seconds.
Andy
July 2007