whooops, I thought I'd set the link for public viewing. It's done now so everyone can see.
https://picasaweb.google.com/1100556...edQuickRelease
Yes, dabac, the part you posted the link to is very similar in principle to my one (a slightly different shape but all the important bits are still there and in corresponding positions). Your description tallies too.
It's actually very useful to know what it's called and where I could get another Locating Sleeve if I ever needed one. It is a confidence booster to know how it comes apart and what to expect, and to know that at worst I can get hold of a replacement without undue expense or difficulty.
Thanks for this.