Nest thermostat-E is a type of Room thermostat and I believe even though after the power cut, the room temperature went to 27°C – the Nest thermostat-E was still set to 15°C.
I assume as the boilers programmer was flashing due to power cut, it could not recognise or take input from Nest thermostat-E.
Interesting problem and something I have not seen before 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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