How can I check my Belling double oven (built-in).Food not cooking properly?

I have a Belling double oven/grill. The fan oven does not seem to cook evenly e.g. chicken not cooked through after ample cooking time; cake still liquid in centre after recommended time plus ten minutes; but temperature seems very hot sometimes, even allowing for the fact that it is a fan oven and I cook stuff at about 20 degrees less than recommended. The oven is only 9 months old.