Mine is a fan oven, but I know it's a bit temperamental, e.g. the front right hand corner cooks faster than the back left, the middle shelf cooks slower than the top shelf etc.
So I'd experiment with positioning in the oven, and beyond that I would do what thisisyesterday suggests and part way through cooking lay a bit of foil or something over the top to stop it catching.
Also, do you always use the same banana bread recipe? Bananas are very sugary fruit, and then with sugar and maybe honey in the recipe, it doesn't take much for it to burn. You could maybe look at different recipes and see whether other ones cook more reliably.
Might be worth considering getting an oven thermometer to check the temperature, in case the thermostat is knackered. Also maybe use a little less banana? Could it be the mixture is too moist, so the outside is burned before the inside can set? That's the trouble with the old '3 medium bananas' thing - very subjective, variable moisture content etc.
I use Jill Dupleix's recipe from Times Online (I think) and have never had problems with scorching. Also, if you fancy trying different recipes, have a look at Orangette - she has several recipes on there with ginger and cinnamon and chocolate and all manner of yummy stuff!