Hello all, the replacement arm/link arrived from chambers and was the same as the one fitted to the gun. So I decided that I could experiment and if I made a mess could just replace the original link.
After 50 pellets then there was a rub mark on the cylinder 3/4” long 1/2” from the piston end of the cocking slot in the cylinder.
The arm/link was being held off the cylinder by nearly 1/8”.**
The roller could not be in contact with the cylinder because .
1. It was not proud of the link and the face of the roller is concave anyway.
2. The arm/link was being held off the cylinder, at the end ( closest to the cocking slot ) it was in contact with the cylinder but at the roller end it was off the cylinder by 1/8”. **

Assumptions are dangerous but I assume that the roller should run on the cylinder?

I removed approx 3/32” from the face of the arm/link so that the roller was proud and 'capable' of running on the cylinder. But the arm/link was being still held off the cylinder.
Bother or words to that effect.
Why, well the wings on the end of the link were at such an angle that they were holding the link off the cylinder. Careful use of a dremel with a fine cutting disc altered the angle and the arm/link sat down nicely with the roller on the cylinder and clearance all the way between the link and cylinder. I hate to think of the stress on the 'wings' and the cylinder slot.

All done, NO, bother double bother, when fitted to the stock then there was an audible 'click' on cocking, freeing off the pivots and sitting the link down created a large gap between the link and a plastic ( PTFE, Delrin? ) button set into the stock, the click was the link 'snapping' into position up against the cylinder.
Drilled hole in buttons rear face and fitted a spring, result!

The cocking is now silky smooth and a lot easier.
A remark from a chap at the range before it was sorted was. “ Blimey that has a kick”.

A Tinbum kit has transformed it into an gentle nudge and it is now a pleasure to use. Dare I say it, as it should have been. I must say it has proved interesting.
Thanks for reading.
ATB.
Geoff.