OK a few more details.
The CH switches off at 1030pm, this was about 330am, an hour or so before it was due to come on (wasn't very happy about being woken up an hour earlier than normal either).
I checked the boiler, timer etc. Nothing else was on, only the pump which I could hear wirring away in the airing cupboard. Even when I set the timer to "off" it kept running, until I finally switched it off at the fused spur.
It wasn't that cold last night and I don't believe this has ever happened before, even on colder mornings so am not convinced its frost protection.
Pump ran for about 10 mins or so until I switched it off the system at the spur.
Not too sure what its been up to today as the heating is still on when I leave and is on when i come back from work, suppose if i get woken up in the night again i'll know.