Microwave buzzing and flashing electrical discharges ?

AEG Micromat MC1231E-M built in microware.

It's still works but with the now added spectacular special effects of a miniature electrical storm.

Have looked inside but nothing damaged, some soot around the transformer (£146 for a new one, ouch).

Given the price of the spare parts I don't want to replace anything based on a guess.

Suggestions please!