No 12v power to Truma water heater

I'm having trouble finding the reason why 12v is not reaching the gas control pcb.
To check/eliminate the heater I connected a 12v battery direct to the pc board and the heater checks out ok and the green light comes on, so the problem is with the 12v supply to the heater. All other 12v systems work.
I understand for 12v to be supplied to the heater the window above the heater exhaust has to be closed and there is a switch on the window which I presume has to be 'made'.
From previous experience, the water system also needs to be up to pressure before the heater will light.
My problem is I'm not sure how this pressure completes the electrical circuit because the pump switch 'breaks' to stop the pump when the system is up to pressure.
Is there a relay in the system?
Does anyone know how the heater/window/pressure switch circuit is wired and where the components are?

Thanks for replying Dave. I have the original AS Duetto manual with schematic diagram (drawing number SAD11543) but being a basic schematic it doesn't show all the components and +/- connections. Hopefully yours might show a bit more detail.
In the meantime I'll temporarily bypass the window switch (to prove the switch) and see what happens.

Thanks anyway Dave. I have recently changed the domestic water pump but the heater problem was there before that.
Will persevere but access is like painting your hallway through the letter box!
Will update when I've found problem.
Bye for now.
Howard

Update - Truma Water Heater now working again and we have hot water on gas. The window switch was working ok but I couldn't find the break in the 12v supply, so I've wired it straight to the leisure battery for now.
If and when I find the breakdown in the supply I'll update and hopefully be able to help others.
All the best to all Auto Sleepers out there.
Howard.