Potterton Kingfisher RSL 50?

The boiler has started to cut in an out far more frequently than before.

Turning the temperature contol up or down (on the boiler) stop it for a while, but them I assume the system reaches temperture and in clicks in an out rapidly again.

Is there a part I can easily replace?

Boiler is about 5 years old

I am loolking to extend house soon which will require the boiler to be replaced so I want to do something to get me through the winter.