when you reset the MCB can you feel it blowing back out or does it just feel loose?

if the mcbs OK it should feel possible hear it blowing out.

next turn off all the switches off on the upstairs circuit, this will illuminate part of the circuit. try resetting the MCB.

If it stays in turn the light on then see which one causes it to trip.

if it still goes off you could start finding the fault if you are OK with electrics, not that you should of course.

You could do this by opening a ceiling rose, and taking all the red/browns out of the centre conector so they aint touching, try turning it on, keep doing this until it resets, then turn off and connect another in, hard to explain really.