Remove mains power. Look at the the bottom of the back panel and find the float. It is where the two tubes go to at the bottom. Fidley and normally clips opened buy removing the panel to the side and comming from the inside towards the back panel. Take of the electrical connections remebering or photgraphing where they go and remove the block with float and pump in it. Clean the whole thing out and make sure the float can move freely.
Trev
November 2012