Neff Combi Oven’s microwave failed

I have a thirteen year old Neff Combi Oven model C57M70N0GB/01whose microwave has stopped working. The failure tripped the ring main circuit breaker. after resetting the breaker the microwave runs for only three seconds before cutting out. Looking inside the unit, the 10A ceramic fuse has blown and replacing it only results in repeating the original sequence (trip, reset, runs for three seconds then stops) and the replacement fuse blows. I am aware from googling the problem that I am looking at replacing the magnetron or inverter or both. The magnetron has no leakage to earth according to my multimeter. I know it will be expensive to replace the parts but I don’t want to replace the whole oven as a new unit would not match the main oven that sits directly below its I am looking to repair if possible. Looking inside the machine gives no clue as to what has failed. My question is: is there a simple way to determine whether I need just the inverter, just the magnetron or whether I need both and is there a risk, should I replace only one part, that the other part could cause the new part to fail?