You will find a K3 will do the trick.
If you can't find one, try Morgans on Chamberlin Road. Aylesbury.
Screwfix won't have a clue what you are on about. You need to proper machine tool dealer.
Andrew
January 2013
gomto this web address and you can get the part
http://www.servotool.eu/index.php?p=item_list&id=TDM4003