Boiler fires when it shouldn't?

Got a Performa 28 combi. I've noticed that sometimes when no heat or hot water are called, the boiler fires up seemingly for no reason.
The heating and hot water lights are not lit but the flame light is on and the heating gets hot. The boiler temperature never seems to get above 30 degrees.
If i turn the power off for a few seconds and back on again everything appears normal, also if this happening and i demand heat or hw, they both work fine and the system returns to normal.
It does appear to be a temperature related problem, in so much as that the ambient temp always drops to around 15 degrees when this occurs. I know this boiler has a frost stat which operates at 5 degrees , could this be the cause if it were faulty? where would the stat be in the boiler? could it be isolated to see if it cures the problem?

Thanks for any advice