Hi Andrew. I don't know whether this site is any good to you as its in the US and I assume there's import laws between your nations??? Also I don't know when the site was last updated. Anyway I done a quick search on their site and this part assembly came up in the results. It shows what looks to be the part your after on the whole retracting slide group. The part itself isn't cheap but they may be able to help you anyway. The address and a quick picture provided below. Hope this helps.
Regards, Nathan
http://www.bmgparts.com/50cal.html
D23800A $379.95US plus shipping.