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The fault in the microwave sounds like diode or PCB power module failure, the popping noise you heard is often associated with a magnetron problem, an educated guess would be the magnetron failed which caused a surge which damaged the diode or PCB, which in turn tripped your MCB.
Unfortunately this doesnt sound like an economical repair, since you have already checked the fuses, the next place to start would be checking the diode with a multi meter, the spare can be sourced from here (if found to be faulty). You cant easily test the magnatron until you have the power circuit working, however the PCB costs circa £150, the magnatron £100, so would advise against repairing the unit if the diode appears to be functioning.