Hi Rellis
I don't know much about the Webley mk3, I've shot one but not owned one. However, a friend asked me to find a sight for his and he ended up getting one from here:
https://www.airgunspares.com/ar77web...tassembly.html
It's a reproduction and I seem to remember him saying he had to alter it to fit. There's also a Parker Hale sight that comes up for sale now and then on auction sites that I think came on target models
I'm not sure the Chambers repro one is worth the money, I would keep an eye out for an original unless others have had a good experience with them ??