Thanks Ian, useful input. 
However, as I already have all the basic wiring in place I'll probably just install a commercial unit anyway. I rather fancy a relocatable wireless thermostat and the potential of remote control.
Quote:
Originally Posted by HPsauce
As far as I'm concerned I could install a modern "all-singing, all-dancing" controller with wireless and remote access and all I would need to do would be to apply a bit of thought to the programming to ensure hot water was on when the heating was on.
Even that wouldn't be really needed, it would be a doddle to fit a relay inside the boiler wiring box that just turned it on when the pump was energised.
(Edit: Having studied the wiring diagrams I don't even need to do that, it's just a matter of rerouting a cable that takes voltage from the pump supply to trigger the boiler so that it will bypass the tank thermostat.)
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My last comment ( in bold) is wrong as connecting the pump supply to the boiler will of course ensure the pump is energised when the boiler is firing but ONLY the hot water is intended to be on! 
But I would also need a relay to make it failproof against bad programming.
Logically, as noted by Ian, you need the boiler firing when either or both of heating and hot water require heat input (as dictated by the respective thermostats). Electrically that does really need a relay to do simply.
Edit: The current wiring (which follows exactly the diagrams in the boiler manual) effectively uses the Cylinder thermostat as a switch feeding power to the boiler from either the hot water signal (cold tank) or central heating pump supply (hot tank) so will work fine even without a relay if programmed appropriately. 
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Last edited by HPsauce; 3rd March 2019 at 05:15 PM.
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