I'm looking to now change the DHW Diaphragm.Could be a number of things but most likely and cheapest to try first is the DHW pressure differential diaphragm valve.
Flip down the flap (?!?!) and somewhere near the middle you'll see a brass disc-shaped object with 5 screws round the edge (there's two - it's the larger one you want). As you open the hot tap, the pin in the centre of this should come out and press against the grey fulcrum arm. This then pivots and makes the microswitch, telling the boiler to fire up. Chances are that the rubber diaphragm inside the brass disc is holed or torn slightly and the pin therefore isn't coming out enough due to the pressure dif being messed up. To test, run the hot tap and pull gently back towards you on the arm where the pin should hit it. If the boiler fires and your hot water is fine, there's your problem.
New diaphragm alone, assuming no greater problem with the valve, is 5 to 10 quid. Easy enough to change - turn off the boiler, turn off the cold to the boiler, drain out by opening hot taps, remove the fulcrum arm, remove the 5 screws around the edge, replace diaphragm, reassemble in reverse order. Job's a good 'un.
There is also a small weep around the pin which pushes the microswitch.
Reading other threads, I think I will have to change the seal around the pin which comes in the Vokera Excell T0019 Service Kit,
which I think is one of the brass sealing nuts (?)
Which other parts should I change while I'm at it in the DHW Diaphragm Valve, which come in the kit ??
Is it just those 2 parts, or are there others, rubber seal/s, white plastic thingy in picture ??
Any other advice appreciated.
Thanks
http://www.discountedspares.co.uk/vokera-t0019-service-kit-boiler
http://www.buyaparcel.com/pageview.php?page=show_product&ecommerce_stockcode=T0019