Screeching engine but no drive to blades?

My Bosch AHS -15 electric hedge trimmer motor emits a violent screech but doesn't drive the blades. I thought perhaps the blades may have seized up during the winter, so I applied loads of WD47 and loosened and retightened the tiny bolts running along the spine. This was not the answer.

Can anyone advise me how to resolve the problem myself, if that were possible? Knowing Bosch, it would be cheaper to buy a new hedge trimmer then send it away to be repaired!