We had the same problem with our Bosch fan assisted oven. The fan is secured very tightly for a light aluminium casting. Using grips or similar to hold it while applying considerable force to the nut risks damaging it. Since the top clip is a replacement item it makes more sense to gently prize this open to enable the element to be removed and replaced. This is not without difficulty but seems better than risking the fan. Make sure the clip holds both parts of the new element and is clear of the fan.
Sharon, March 2006