Ok, so basically, the installer assumed blue wire was neutral, L - N was approx 10Vac, N - E was approx 248VAc, L - E was approx 248Vac, so originally I assumed reversed polarity.
What was actually happening was blue wire in spur was switched Live, so boiler would only work with the old room stat that was left connected.
The stat was obviously left in a position where the boiler would operate, and was either turned by customer (said it hasn’t been touched), or it wouldn’t work, as temperature was high today.