Just for anyone who was interested, I took the faulty MCB in bits, and it was the solenoid pin on the magnetic trip which had got stuck, preventing the MCB from being reset.
Whacking the bottom of the MCB which I haven't opened onto a table top has got the pin to go back to where it should be, and it will now hold in.
Whacking the bottom of the MCB which I haven't opened onto a table top has got the pin to go back to where it should be, and it will now hold in.