Microwave has always worked fine - blew a fuse - replaced it and worked good for a while - the fuse blew again - the question is what makes the fuse blow when the door is opened ?

If fuse blows when door is opened I would suspect a short in the wiring to the light.
Suggest you remove the bulb and see if you continue blowing fuses. If wiring to bulb is faulty you may or may not continue blowing fuses. If you no longer blow fuses that would confirm faulty wiring to bulb.

If you open up the microwave, be careful. That capacitor can hurt you even when unplugged.