@mobiusnz since it still heats, just not as effective as it should, I am doubtful it is the magnetron. I am not saying it is not but test the other parts first. check your capacitors as well as your high voltage diodes. As always, when working with HIGH VOLTAGE, you always have to be aware off that you know where and what you are doing. Always discharge your capacitors before touching anything. Use insulated tools only and stay away from touching anything with your bare hands. It's just me being careful :-)
On your inverter board, check the capacitors for any leakage or other damage. Check for anything that might be looking "odd". Post some pictures of anything that looks questionable. I am also a bit leery about "the body of the microwave is getting quite hot" Ones you have it open, take a good look around for any bad contacts that may have arched and are not properly connected. Again, use caution :-)
See if this [document|39584] can help you to fix your MW.