Sharp Microwave Oven (Sheringham NR26)

This is a Sharp R-4G57(B)/(W)M Microwave oven with grill, and comes complete with turntable and instruction book, It is rated at 850W. It is about 20 years old, but still seems to work OK - we've been using it regularly until a week or 2 ago.
The external dimensions are : 520mm wide, 305mm high, and 413mm deep. The internal dimensions are 342mm wide, 193mm high, and 368mm deep. The capacity is 24 litres.
However, the interior lamp has failed, although I would guess this must be repairable. The inside of the door is showing some rust marks.