More zones please?

[Milleniumaire]

Do you have the Wunda control box, which controls the UFH pump and powers the UFH zone valve? If so you may wire a 'standard' roomstat to it, and power the actuators from it.

No, there isn't a Wunda control box in the UFH setup. I guess removing the VR 81 from my existing setup and using the Wunda control box with non-vaillant thermostats would be an option, but as the VR 81 is wired into the VR 61 I assume the use of the VR 61 for the UFH (current and new) would be completely removed?

I would then have a system where the radiators where working off the VRC 430 and VR 61 and would use the heating curve/weather compensation (essentially my original system before UFH was added), but the UFH zone (for the existing kitchen and new bathroom) would simply be controlled by a couple of third party thermostats.

 

[MeldrewsMate]

It looks like you have a solution. As said before, the UFH operation will not be enhanced by weather compensation.
I think Danfoss (prob others too) do RF thermostats with a dual RF receiver, but as I prefer the non-battery devices I suggest two wired programmable stats for the UFH.
Honeywell Lyric T6?

 

[Milleniumaire]

How far in metres would the pipe run be from the existing manifold to the bathroom?

A rough measurement suggests the pipe run would be about 13 metres so connecting the bathroom upto the existing manifold should be doable.

I'm going to contact Vaillant, just to see what THEY might suggest then I'll have a chat with the heating engineer, assuming I can get hold of him at this time of year

 

[Dan Robinson]

Their controls are expandable, but at a cost and added complexity.

My solution to simply connect it to the existing manifold would be much more cost effective and if you truly drill down into your heating habits/needs, perfectly useable.

Otherwise, as I said before, Evohome is a pretty straightforward way to control the lot easily.