Internal circlip of some sort ?

[Adam Funk]

That's true, it's what you always see in distillery tours!

[Jethro_uk]

Thanks for all the suggestions.

Asking here seemed to have worked, as I have just had an email from the suppliers apologising for the shortage. They've had some issues themselves, but are going to be able to resupply in time.

That said, I shall keep the old housings, in case I need to DIY it in future.