its 1 of 3 things, your pumps knackered, the motor has split its bearing seals and lost its grease or the condensor for the motor has worn out. It's a Riello Mectron burner on this boiler or at least it should be! So, a pump will be around £50 same again give or take for a motor and the condensor which is not part of the motor (seperate part) is about a fiver. Replaced countless ones of these in my time. I recommend getting an OFTEC engineer to do this work as there are parts that can be a pain to remove, plus, if the pump is replaced, the boiler will need to be set up again, ask about a service at the same time.....might save a few quid.
Ben the Oil Man, October 2008