Whirlpool Gold Accuwave microwave not heating correctly

Our microwave has stopped heating efficiently. It was heating fine up until a couple of days ago then it kind of stopped.

I have taken it down, found the service instructions and tested out the magnetron, magnetron thermostat & 20A inline fuse. All checkout as per the instructions for testing.

When i put a glass of lukewarm water in, set the microwave to 2minutes on high power the water temp only increases a little.

Same glass back in for 5 minutes on high power and the temperature only increases slightly, I would expect the water to be close to boiling temps after that time.

I need to know if there is an issue with my magnetron slowly dying or if the power inverter needs replacing & what testing I need to do to confirm this.

Thanks