I installed the glow worm one from cityplumbing and it's working fine work my vrc 720f/2, less than 1 minute of effort.
The mounting bracket was also an exact fit for my existing vaillant unit, so simply dropped it in and I was done.
I've found the Glow Worm sensor at cityplumbing for £65 and a Vaillant VR20 sensor at theheatexchange for £143. Assuming these are interchangeable.
Anyone tested the Glow Worm one with a sensocomfort VRC 720f/2 (mine's talking to a talking to a VWZ AI heat pump control interface)?