After serious thinking Terry Casey wrote :
There is no check sum but there are three parity bits, one on
the minutes, one on the hours and one on the day/date data.

https://en.wikipedia.org/wiki/DCF77#...interpretation

Well I knew that, I just class it all as a 'check-sum'.

The clock's data became corrupted a second time too, Sat/Sun.

I carried out some more tests yesterday, on the HW heating, setting it
run a legionella cycle. That caused the boiler to run 85C, which raised
the HW a few degrees above the 50C, but the SDM 1901 actuator then
began cycling. Radiator/house temperatures increased due to this valve
cycling. So my best theory now is that the VR 65 is not compatible with
a Sunvic SDM1901 actuator, but Vaillant offer no clues as to what
actuators it should work with - they just state a 3-port mid position.