Thanks to this thread I now have hot water from my Halstead Ace High! Cheers everyone :thumbsup
Procedure:
Turn off power to boiler and water at main stop tap.
Drain hot and cold water by opening all taps. Drain CH system (I just removed the bleed screw from the lowest radiator in the house).
Remove top screw to release front cover.
Remove top left and right screws on control panel and drop down.
Remove two brass screws that hold the disc shaped housing (the diapragm is inside this).
Snip the two zip ties (if they're there) that retain the clips over the the pipes entering the housing underneath.
Remove the two clips and then the pipes.
Remove housing.
Remove 6 screws on housing and separate two halves of housing.
Replace diaphragm (and valve if included using spanner).
Reverse the procedure applying a little grease if needed to the two pipes to make it easier to get them in.
Dead easy, took 10 minutes. Cost £18 from local shop (genuine part!) but available on Ebay for around £4.50.
Parts/tools required:
New Diaphragm
Phillips screwdriver
Large(ish, I used 6mm) slotted screwdriver
10mm spanner if replacing valve
Tiny amount of grease (if needed)
DaveyM69
February 2011