@julianc, it's very pricey – we're in the process of doing the maths and trying to see how things stack up in terms of costs and CO2 emissions. 

Do you think at some stage will have to improve the insulation/draft proofing and invest some money in your ashp setup where you already know there are issues? 

Can't be an easy decision working out where to invest but i assume it should be possible to run your ashp alone more efficiently with further investment? I do feel for your situation. 

the schematic on page 118, suggests that the EHS has no direct impact on DHW, and access to it is via a buffer tank. That flies in the face of the govt report which highlights DHW is best done by traditional heating source (oil in my case), but that doesn’t look like an out the box solution from Grant. Will speak to them Monday

In particular, 8.3.2 describes 'Backup heater in Supplementary mode' where it'll run both at the same time, using the oil burner to boost up the temperature when the ASHP can't do it all.

Note that the heatpump has its own room temperature compensation/control system which Grant completely ignore by using classical on/off thermostats as if it's a traditional boiler. It is possible this supplementary mode doesn't work if you use them as Grant indicate.

I don't have a hybrid setup so no direct experience.  In general, when people talk about hybrid setups are they running both at the same time, or just turning off the heat pump and firing up the boiler?