what causes the sabre he 25 to come on, on its own?

RTFM.

30 seconds with Google leads to the manual here:

http://www.sabreboilers.co.uk/Sabre_HE_users.pdf

See para 1.3.1 MODE SELECTOR SWITCH AT: OFF:
When the mode selector switch is in the ‘OFF’ position, the appliance will not function for hot water or heating. However the appliance will still operate in frost protection mode, and if unused for a period of 24 hours, will momentarily activate the pump and valve actuator to prevent seizure due to non-use.

So it'#s probably either frost protection or anti seize measures.

X
April 2015

Some installations have a frost thermostat to turn the boiler on so that the pipes don't freeze up - this may be your situation.

I think 4°C is the typical threshold.

This is just a guess...
April 2015