That's odd that they never mentioned that when I called a couple of hours ago. I have NOW spoken to a clerk who knows what he is talking about. He advises me that -
1. The Pedersoli backsight is sold as one unit - no component parts are available. You can order it from Kranks, less the 20% sales tax we have here.
2. The old sight, whatever it is you have, would need removing in toto, and the new sight replacing it. This IS a gunsmith job - the original was brazed on. OR, you can cannibalize the new unit for the only part you need.
3. I can send you the dimensions of the slider from my own Musketoon, and you can get a shop to make it for you. Might be the cheapest of all options.
BTW, the craftsmanship of the Pedersoli version is nothing less than appalling - the finish is not worthy of the word and it looks like nothing more than an oil-blackened casting. and £104? You have to be kidding. By way of contrast, the P-H version is a little work of art, as, indeed, the original was in its time.
PS
@BisleyJohn - the diameter of the barrels of both guns is identical at that point, having looked at and measured them both a while back in a comparison.