Hi, now everything is SPD's I've been told they need to be on their own MCB.
It depends on the SPD. They all need some form of ultimate backup protection but for many that can be a fuse in the 100-300A region, so for those the supplier's fuse is in fact enough.
For smaller SPD then you are correct in that they specify something lke a 32A MCB "just in case"
What stops the MCB tripping first if there is indeed an overvoltage surge?
Most surges do not pack enough energy in to the 20-200 microseconds to trip the MCB's coil. While you see a typical MCB like a 32A B-curve as having the "instant" trip at 5*32 = 160A and your lightning surge could be in the kA region, you have to remember the MCB's idea of "instant" is a few milliseconds, where as the lightning surge is 10 or 100 times shorter, so higher current is needed for that to deliver the same acceleration to the MCB's actuator.
You get something similar from simultaneous switching of a lot of LED lights, they can have quite high inrush, but not for many milliseconds, so it can often peak at a couple of times the "instant" trip and not trigger the MCB.
Isn't this ADS of the thing designed to deal with the fault, surely the SPD as a sacrificial part wants to be connected to the lightning strike, not immediately protected and disconnected by the MCB.
Sort of. ADS is all about disconnecting on unusual currents, either overload (fuse, MCB) or differential (RCD) or ether (RCBO) that imply a fault, and so the person in contact has a low exposure to shock risk. Related to that of course is the overload/adiabatic risk of fire.
For the SPD and its related protection it should be designed so that most surges that are within its safe handling do not trip it, where as if too much energy is applies it fails as safely as possible. Of course that also means down-stream equipment is likely to be damaged afterwards.
Away from the supply fuse/MCB aspect, SPD usually have melt-able connections so if they do overheat seriously they open circuit. This might be from a massive surge (e.g. direct lighting hit on the building or supply pole and SPD not rated for that) or it could be from a supply cross incident when you get a big over-voltage for a short time due to falling HV cables, etc.
TL;DR if the system can survive the surge then the MCB should not open. But both the MCB and the "SPD failed" windows should be regularly inspected in case they didn't.