If it's just a leak you don't need a new diverter valve (typical cost £80) you need the washer/o ring kit. Though this is usually unobtainium. Some diverter valve replacements include the kit but some don't.

You can get the real macoy from HRPC, or you can get cheap supplies from http://www.ezypart.co.uk/index.

This is a nightmare job for the uninitiated and an nasty one for those of us trained to do it. Round here we charge £100 labour for that job and some days we wish we hadn't started it.

Usually when they go wrong it's only the diaphragm, but the stupid manufacturers build them in such away you have to remove the diverter valve (in most cases) to get at the diaphragm.

You have to isolate flow and return and cold feed. Open blow off valve manually to drain boiler, dump the remainder on the floor around boiler while you undo nuts and it gurgles out, catch what you can but have a few towels ready. If yours has o ring seals and you can't find replacements sometimes high temperature silicon grease might be sufficient. Where there are fibre washers you really need to find new ones, but if you can't you could try making gromets with ptfe (wrap it around finger). Or the time honoured cure all boss white and hemp.

effing good luck you'll need it!