Sime super 90 - Water only heated when tap on full?

I've got a Sime Super 90 boiler and have started having problems with it. The central heating works fine but the hot water only ignites the boiler when the taps are running more or less full.
This means that the shower isn't drawing enough water to trigger it and the only way I can have a hot shower is to turn the heating on and have a tap running at the same time.

Reading around I think it is probably the diaphragm that has gone? Is this right? If so how easy is it to replace myself or should I get a professional in? If I do replace it myself where is the best place to get the part?

Many thanks