Update: I have since found some more help and have now been able to remove the timer from the wall.

For info. There is a small slot about 10mm X 3 mm on the lower RH side within the groove that runs all the way round the timer. Insert a flat screwdriver into this slot, push upwards to release the "finger catch" that holds the timer to the back plate. The timer can then be tilted upwards and away from the wall.
Regards
Andy.