I had this problem some time ago. The big metal fan at the back stopped working - the motor would sometimes have the strength to turn the fan, but other times it would turn slowly or not at all. Giving it a bit of a nudge through the grill at the back will determine whether this is the same problem because after a bit of encouragement it started working again.
After taking the oven out of the enclosure (two screws holding it in) I placed the oven on a table pulled close to the enclosure (you are restrictred by the length of the power cable). I took the back off and had a look at the motor which was very gunked up with general oven grime. It is quite difficult to get at to clean, but I got a toothbrush in there and a long pipe cleaner wrapped around the spindle. You can remove bits of the motor, you can even unscrew the fan from the inside of the oven, but I couldn't find a way of taking it off completely.
I would recommend taking the oven and grill doors off before moving the oven out of the enclosure, otherwise the oven is very heavy.
Paul V, November 2008