Check with the plumbers, but the following might make you go "aha" - they are all things I have noticed with my Drayton wireless programmer.
Yes, OK - different make but who knows....
Going to a strange temperature
Have you been overriding it in the evenings?
The way the Drayton Digistat works is to store not the temperature you want at each time period, but the delta between one and the next. e.g. say that overnight you have it set to 18°, going up to 21° at 06:00, and one day you get up early and think "**** me it's chilly", and manually take it up to 21°, then at 06:00 the programmer thinks "right-oh, time to raise the temperature by 3°", and takes it up to 24°. Maybe yours does the same.
Losing communication
I've noticed the following:
Power cuts screw up the comms, often requiring the "relearning" process to match transmitter & receiver to be gone through again.
The batteries in the transmitter go flat enough to cause random loss of communication long before the transmitter decides that they are flat enough for it to put the low battery symbol up.
ATEOD though yours is still under guarantee, so maybe get it replaced?