Hello Gordon,
I have a feeling that the top nylon gear is split, this is a common fault on all 400 models, the nylon gear drys out over the years and breaks up, take the top cover off and round about the middle of the machine you will see the two nylon gears, look at the vertical gear, you may see the teeth broken away, the gear will probably look dark yellow now, you can get the gears and you can fit it yourself if you are a bit handy.
Ken
drivewithken@blueyonder.co.uk
October 2007