Replacement plastic threaded nut on Aqualisa aquarian cartridge?

Hi, can anyone tell me where I can get a replacement plastic threaded nut that secures the spigot end on an Aqualisa aquarian cartridge please? The nut is split, but I'm trying to avoid having to spend £100 on a replacement cartridge for a plastic nut that must cost less than a couple of pounds. Where do the old cartridges go? If I can find an old cartridge, I only need to undo the retaining nut. Any help/advice would be greatly appreciated, thanks.

Phil Gould
May 2012