Boiler running when Timer is off

[richchad]

I have a Glowworm Fuelsaver 80 Complheat boiler linked to a Megaflo system.
At 5.15am over an hour before timer switches on central heating I let the dog out and noticed the boiler was running, no flame, so assume it was just the pump, timer hadn't switched on.
I do not know how long it had been running?
I turned timer on, boiler lit okay, turned timer off and all stopped.
Any ideas?

 

[baxpoti]

Frost stat

 

[richchad]

Where would the frost stat be located?

What does the frost stat do.....the boiler wasn't fired up just either the fan and/or pump running?