Given the reported chain of events, i.e. grid power surge, after which the MCB trips when the space heater is turned on but not when its turned off, the logic would point to the power surge causing damage to the space heaters control system rather than the Nordelectric unit. But also given the historic reports of problems with the Nordelectric units, and cheap MCB.s fitted by some manufacturers that diagnosis is not certain.
You may be able to check if its the heater or the MCB, but it will depend on how your caravan is wired. If the MCB on the heater circuit also feeds a 13A socket then:
Find a largish load such as a 2kw fan or panel heater
Turn the caravan heater heater off,
and plug the 2kw into the socket on the MCB circuit.
Now try the MCB
If the MCB still trips with new 2kw heater than it points to the MCB having become too sensitive and it needs to be changed. If the new 2kw load doesn't trip the MCB, then it points to the caravan heater having been damaged by the power surge.
If the power company admits to having caused a power surge then you should be entitled to to have the damaged equipment repaired or replaced. -contact the power company.