It's probably a loose connection at the magnetron or a bad magnetron.
If it's less than one year old, it should be fully covered under warranty.
I'd suggest contacting the manufacturer to get it fixed free.
If
it's within the magnetron portion of the warranty (usually 5-9 years)
but out of the original full warranty (usually 1 year), then your
manufacturer will cover the part cost, but not the cost to visit you or
the labor charge to do the work.
Some manufacturers (in the US anyway) will send you the magnetron to do it yourself. If
nothing else, you can save about $75US by taking it to the shop yourself.
Apparently you're in the UK, judging by the model number.
You can find Sanyo UK microwave owner's manuals
here.
You can find Sanyo UK support, parts and authorized servicers
here.
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