Calibration wrong on Gas Cannon Chichester Oven?

We moved into a new house two years ago with the above oven so I'm not sure how old it is. It frequently undercooks things so I tested it with an oven thermometer today and the gas marks on the dial don't correspond with the correct temperatures. eg - dial says 4 - thermometer says 150 C, 5= 160, 6= 180, 7= 195, 8= 210. It should be 4=180, 5=190, 6=200 etc...

Is there any way I can try fixing this myself, or do I need to get an engineer out to look at it?

Thanks, Damian.