I've just had exactly the same problem and just sorted it. Here's how.
Symptoms:
Heating comes on fine but hot water (e.g. for shower) does not. Problem starts out being kinda intermittent but then starts happening all the time. Initially can be solved by turning on the central heating to get hot water for the shower but eventually even that no longer works.
Problem:
Tear in the hot water diaphragm. An intact diaphragm will tell the boiler when a tap has been turned on as the pressure on one side will be different to the other side. A tear in the diaphragm means no pressure differential so the boiler does not fire up when a hot water tap is turned on.
Solution:
1. Get a new hot water diaphragm for a Halstead Finest Gold Combi Boiler (gas council # 840-505). Cost approx £30 from somewhere like Gasworld in Acton.
2. Drain the boiler, turn off the mains water & electricity to the boiler
3. Remove the top panel (screw on the top surface) and be ready to unscrew the panels on the bottom surface so you can get access as needed
4. Take pictures of the boiler so you can remember what it looks like when you're disassembled the following pieces
5. See the 2 parallel silver pipes with some 90 degree bends in them going left to right across the bottom of the plumbing at the bottom - cut the ties on the right hand side of them where they go into a disc-shaped brass object which is side-on to you & pull the pins out, probably breaking a red wax seal
6. Unscrew the flat L-shaped plate at the left-end of these 2 pipes
7. Pop out the 2 silver pipes (some water will spill out but not much) - mind the o-rings at both ends of both pipes - if these bust you should be able to get new washers at any B&Q etc
8. Unscrew the 2 (vertical) brass screws which are holding this brass disc in place and just pull it off the left hand end of the spindle
9. Unscrew the approx 6 or 8 screws around the outside of this brass disc, lessening the pressure around the perimeter gradually and lever the 2 parts of the disc apart
10. You'll see the 2-part diaphragm inside which should look exactly like the replacement part you bought earlier - there's probably a small tear somewhere on the black rubber diaphragm
11. Replace the diaphragm & re-assemble, making sure you pop the silver pipes in as far as you can else you'll have some water squirt out when you turn the mains water back on
12. Be careful of the water pressure gauge - I busted mine at some stage during this process - I think the wire/tube at the back of it is fragile
13. Send me a cheque in thanks for saving you £75+VAT+£30 parts!!
Paul S
April 2008