Baxi bermuda 552 back boiler- Pilot works but Burner not igniting? ?

Hi- any assistance would be great!! the boiler has not been serviced in a while and is not providing heating or hot water as the pilot is working but not igniting the burner- when the clock timer is set to ON and the Thermostat set to a high temperature so as to call for heat. The hot water is gravity driven to the cylinder in one of the upstairs bedrooms

Anyone had similar issues with this model? I live near Heathrow Aiport- can anyone recommend a good plumber, if I can't do myself?

Cheers