So, here's what happens with it connected that way...
With the heating and hot water turned off at the Drayton controller, obviously all is silent. If I go to the Nest, it has a message about not being able to control the heating (as expected I guess).
If I turn the heating to constant on the Drayton, I hear the boiler immediately spring into some sort of life, but there is still no connectivity at the Nest because it needs to be paired, which I have to do each time I turn the heating on/off at the Drayton.
Once I pair the Nest and turn the heating up on the Nest, I hear further activity from the boiler.
This is what makes me think the hot water is essentially performing as though on 'constant' despite being turned off at the Drayton, once I turn the heating on at the Drayton. Then when I get the Nest up and running, that controls the heating as planned, I hear the boiler running constantly until temperature is reached.
I think the heating is working sufficiently, it's the hot water that seems to be playing up.
Then, if I turn the heating down on the Nest or it reaches it's target temperature, I hear the boiler stop, but it seems to continue firing up every 15-20 minutes in spite of this (although I haven't proved it, I think it's what the boiler would do if I ran the hot water on it's own in it's old configuration with just the Drayton, whether that's right or wrong).
To be clear, this house is antiquated and only a 1-2 year thing for us. I was just hoping to get the Nest connected because I have it, but perhaps I need to throw in the towel with it?