how can i remove fan from back of neff B1593 oven?

The fan in our oven has stopped going. I think this is because it has got mucky (sorry for the jargon) and finds it hard to move. Before this happened, it wouldn't start unless you reached through the back grill and tapped it with a knife.
To fix this, I intended to take the fan off and see if the nechanism was mucky. I took the back panel off and am presented with the fan with a bolt in the middle. I tried to undo this bolt, holding the fan stationary to do this. However, despite a fair amount of force, the bolt remains stubbornly in place. I don't want to overdo it. Am I doing the right thing? Does it just take a bit more muscle? ....... or should I call a professional?