No hot water, only when central heating on.

I have identified the problem as being the diaphragm plunger is not coming out far enough to depress the microswitch. i have temporarily fixed the problem by inserting a piece of cardboard between the plunger and switch. i have located a new diverter valve diaphragm and would like to know how hard it will be to replace it myself. does the sytem need draining on this model?
How much should i expect to pay if i get an engineer to do the job?
any help will be appreciated
Thanks

John, May 2007

did one yesterday john. dead easy swap for swap. make sure correct sdiaphragm as no serial number evident on boiler

lc, January 2009

Yes, you need to drain the boiler hot water circuit and the primary heating circuit. Unfortunately, if you have to ask if it needs to be drained, then you probably shoudn't try to repair it.
There are some jobs best left to the trade. It's not everyone's cup of tea.
There are several versions of the diaphragm available. Some come assembled already on the pin. Others(the better kits ) come with pins and "o" rings and circlips to fully rebuild the valve. This usually makes for a better job.
You can expect to pay around £70/£110 for a repair(including parts) or£165/£220 for a new valve fitted.
It varies depending on how well the job goes and where you live.
Hope this helps . I don't mean to be a d.i.y kill joy but you should see some of the disasters i am called out to.

T.h.s, May 2007