I'm feeling a bit chuffed with myself. We had an expensive combination oven that was especially useful for proving dough as the oven temp started at 40º but one day the microwave stopped working. It's been gathering dust for a few years while I waited to justify the cost of fixing it - assuming it was the magnetron so call-out fee + part = more than a cheap microwave.
Anyway, time to decide whether to chuck it away
Why not have a quick look to see if there's an internal fuse blown.
Ignore the dire warnings about high voltage and no user serviceable parts - any scary capacitors would be discharged by now. There's one fuse and it's on the PS, so it can't be that (check it anyway - fine). No obvious signs of magic smoke escaping. Look up the magnetron part no. to see how much they are, but they're only available in Australia, there's a comment about magnetrons being open circuit when knackered. Time to check with multimeter - hang on, this doesn't look right, the connectors are not fully in, they are held up by their insulators! They must have gradually shaken loose by the fan or something.
Put it all back together, and it works perfectly! Apart from the controls needing a few hundred turns to sort out the contacts...