Turns out I don't need a relay after all.
For anyone finding this in the future - I looked at the wiring going from the vaillant hardware to the heating manifold hardware (uf8 rf) and where it connected to the UF8 rf there were ports for C, NO and NC. I moved the wire from normally open to...
I have a vaillant heat pump for my DHW and underfloor heating. The underfloor heating is controlled by Heatmiser thermostats with a UHF-8 wiring centre. I've just upgraded the heat pump controller from a VRC700 to a VRC720 (sensocomfort).
Now, whenever the heating is "off" on the thermostats...