Hit and miss temperatures ? How long to cook, etc, etc

Had a JML Halowave for two days . . . Great features, until I bought an oven thermometer for £3.99.

Oh dear, when it was at 200C it was only showing 130C on the bottom rack. At 250C it could not crawl up to , wait for it, 180C !! Under cooking by a long way.
Took it back and exchanged for an Argos own make Cookworks digital for £44 ..

Result ? Absolutely brilliant ,!! I now have to lower the temperature by an average of 20 degrees, using my oven thermometer every time.
Food is wonderful, cooked to perfection every time. Happy, happy me !!