Confounded GSO focusers! 
Since posting this ad, the GSO dual speed focuser on the ED100 scope has developed the blooming "GSO jitters". There appears to be a way to fix this, but it means pulling the damned focuser apart, which I don't want to do. The focuser still works, but the jerkiness is unwanted.
So I've dropped the asking price by a bit more than the price of a new GSO focuser and to cover the cost of posting the new focuser.
New asking price is
$575.
SOLD
Many thanks for looking,
Alex.