I a similar problem, a small clunk, then a rush of bubbles surging thru' the system and finally a massive banging noise, like someone hitting a radiator with a hammer. Pump overrun stat, goes in the back towards the bottom on the right onto a pipe coming off the heat exchanger. You need to take off the upper, outer cowelling, then the inner cover to get at it, costs about £15.00. Just unscrew old overrun stat and replace, only a few wires. Make a note of existing set-up or take a photo for reference if you really need to but it is very, very simple and obvious once you get the covers off.
DainO
November 2012