Combi HW issues, please help

Hi All,
Not hugely experienced specifically with boilers, but trying to work out if the issue I'm having is an affordable one, or if I'm going to have to consider a new boiler.
Glow worm Compact 80p pilot
Basically, CH behaving itself as normal (not that I need it this time of year), but when I open the hot tap, the HW heating doesn't kick in. In fact, if the CH is running, and I open the HW tap, the boiler cuts heating immediately, as opposed to flicking over the heating circuit and continuing to run the heater as it normally does.
If I turn the HW off, the CH kicks back in and starts pumping/heating again.
I've read a number of things online about issues with the 3-way/diverter valve on various models including this....I think I've located it (can't find a manual for mine online anywhere)...Thing is, if what I've found is the correct thing, it's tucked up behind the water pump in an almost inaccessible position. Incidentally, I turned the power off, disconnected the plug to this part, powered it back up, and THIS time, with the CH running, turning the HW on did nothing (i.e. CH carries on heating as if I've done nothing). Not sure if that helps anyone confirm I've found the right piece of kit.
Any thoughts anyone? Think the boiler is about 11 years old, so it's had a reasonable run, and it had a new PCB a year ago, so it would've been nice to get a bit more life from the old girl before I have to shell out 1500+ quid replacing it.
If that is the faulty part, although I'm fairly handy, I may still have to get someone out to sort it, so any rough price/availability from anyone would be good too.
Thanks for any help/advice you can offer.
--
hodg




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hodg wrote:

Will it give HW in DHW mode?
Is the pilot light staying lit when you try for HW or is it cutting out?
Who fitted the PCB? - you do realise that this isn't really DIY as some settings may need to be adjusted IE the potentiometer
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