I have now fitted 5 Rondostat 20E's digital TRV controllers, bought from Klimalex in Germany. not cheap at 38 Euros each (or 108 for three), plus tax and postage, and I will have great difficulty getting the costs back in savings. However, because I have recently had a Viessman Vitodens 200w fitted, they give me the local room control which is not available with the standard Viessmann control panel, (it has weather comp and no room thermostat).
For anyone thinking of fitting Rondostats, they work very well, perhaps the only drawback being that they only have two on/off periods per day, sufficient for my needs for bedroom and lounge.
Further, the units come supplied with adapters to fit several Danfoss TRV's, but none fitted any of mine, which were all of different types. However their site offers the units supplied with 8 other types of adapter (none were of use to me).
To resolve my problem, I bought Honeywell Angled TRV bodies, as used with their CM Zone, which I guessed would take the Honeywell Rondostats, and so it proved, adding a further £8 or so to the cost of each unit. However, the finished setup is extremely satisfactory, and I am at a complete loss as to why they are not available in the UK, or as Honeywell curtly told me " Not supported"!
gcol, the OP, was very helpful in my project.