If you are undoing the screw on the foot control of which there is only one and it is not unscrewing, it was a ver common fault for the screw to turn inside the plastic case and not undo, if you unscrew it slowly and gently prise apart the top and bottom you will find it will undo, you will need to glue the control back together.
Ken
You can e-mail me if you wish, drivewithken@blueyonder.co.uk
Ken Dourof
March 2007