Belling platinum DB2?

From day 1 - the oven doesn't get very hot, great for casseroles, rubbish for cooking roasts especially Turkey's. Called Belling out - they heated it up for 20 minutes declaring it fine... Christmas came, took 6 hours to cook a small Turkey, in fact a vet could have got it back on its feet.
So whats wrong? Reluctant to call out Belling again?
David