Thanks for the info Richard. I think (?) the SMK dioptre is the same as the one Air Arms sell?
But main problem is solved thanks to Zooma (Bob) and his dioptre set are now sitting on my LGR. Thanks Bob.
But I am now wiser regarding sight compatibility: Out of interest, I had an opportunity to try a Walther Sport rear unit (at least that was how it was described to me) but hey ho .. it would not fit the LGR. The dovetail grip was a thumb wheel tightening the rails by, for want of a better description, deforming them so that they gripped. It was impossible to get the rails to clip on the LGR in the first place but with a little persuasion and 'springing' them they did go on. But it was then not possible to tighten them because the part of the thumb wheel that threads onto the cross bolt to tighten the sight on the rails was touching the top of the LGR mounting on the cylinder so when you turned the thumb wheel, the complete sight would move on the dovetails. Again, so much for standardisation as it appears even Walther were not consistent.
So ... still learning. What fun; but I can see it being a bit frustrating for dedicated 10m target shooters. Maybe the standard 11mm rail air rifle shooters with scopes have it easy.
Cheers, Phil