Neff combi microwave has developed a fault. Oven working ok, but microwave cuts out almost immediately, displays error code E305, bleeps 6 times, then resets the clock.

I've not stripped it down yet, so ignoring loose harness connectors or duff internal fuses, does anyone know what the likely faulty part might be? Or how I'd test to establish which part needs replacing? Don't particularly want to buy bits and find hey're not needed.