Does the sight have a brand name stamped on it?
I've owned a few early Parker Hale rifles over the years but cannot remember the brand of the folding rear sight used on them.
Similar sights were made with the Lyman and Marbel makers names and a search on US and UK ebay may bring some results.
Something like this.
http://cgi.ebay.com/Lyman-Rear-sight_W0Q...%3A1%7C294%3A50
This one is a bit knocked around, but there is also a 'new in packet' Marbel folding rear sight there, if you do a search.
Some other members here own Parker Hale rifles and may be able to identify the brand.
Best of luck with your search.