Hello Neil,
I have a feeling that either the top gear or bottom gear has broken, take the top off the machine and look at the nylon gear directly in front of you, turn the balance wheel by hand and both gears should turn, if the bottom does not then you need to replace it, if the top gear is okay then take the bottom off the machine and look directly under the raceway area where the bobbin case is and you will see two gears, turn the balance wheel and see if the gears move, the gears are quite easy to get even on ebay and I have full instructions for replacing and retiming.
Ken
drivewithken, January 2010