I finally managed to finish reassembling my modified BSA Standard, it has what Ed Bear identified for me as Mauser K98 sights fitted.
It also had some tapped holes in the air chamber where a previous owner had some type of scope mount fitted, this seems to have failed, the holes were distorted and the piston has score marks from protruding screws

I'm no engineer but looking at the thickness of the cylinder wall and how the previous mount had failed I thought that even re-tapping and mounting directly from new screws would be dodgy and probably fail again

A better idea I thought would be a saddle mounted rail butting up against the trigger block to stop any recoil movement using the existing holes to centre and hold it in place without putting any real strain on the screws

My only problem now was how to get it done
Thanks to this great forum's many helpful people I contacted "UK Neil" and he made and fitted a terrific rail mount on the gun. I tried it out yesterday with an old Bushnell 4x32 and its shooting very well and accurately. Thank you Neil.

2 pages of pictures here:
http://www.pbase.com/davecthomson/bsa_standard
Picture of the rail mount:
http://www.pbase.com/davecthomson/image/122092953
Picture of the mount with scope:
http://www.pbase.com/davecthomson/image/122093019
Picture of the finished rifle:
http://www.pbase.com/davecthomson/image/122093045

Thanks for looking...Dave...