I suspect thermocouple needs replacing.An easy diy job, but can be a bit fiddley to get at the thermocouple fixings without removing the burner tray (which should only be done by a qualified (CORGI) person as it involves separating gas pipe unions.)
A new thermocouple should be less than £10 so if it fixes it, it cheaper than calling out an engineer..... and if it doesn't........ well it was worth a try.
Bert, January 2007