John Rumm was thinking very hard :
https://www.vaillant.co.uk/downloads...1-1-931512.pdf

It also still has a connection for an external temperature sensor -
suggesting it ought to work with the weather compensating controls. If that
is true, then it must support split temp operation...

Yup:

https://www.vaillant.co.uk/for-insta...ats-22272.html

Says that it works on all current models.

Thanks, you are correct - but I wonder why their help desk didn't point
me in that direction?