The side of my microwave set on fire I was microwaving a potato cake, which definitely has no metal inside it (or any which would cause a fire), it was in the middle of the platter when suddenly the side of the microwave caught on fire, there's a small piece of what looks like metal built into the device, but I fail to see why it wouldn't have caught fire the hundreds of other times I used it.

Should I be replacing this microwave or is it a one off?
If anything that is actually part of the microwave caught fire, of course you should replace it.
(Theoretically you could have it repaired by someone knowledgeable, but repairing a microwave is not cost effective.)

Continuing to use damaged electrical equipment is asking for a fire or damage to your electrical systems.

There is no such thing as a 'one off' fire that you can neglect.