Thanks for the advice, but that hasn't worked either.
Spent 8 hours now trying to get the seal fitted to the piston & all I've managed to do is bend my valve spring compression tool & ruin the seal as it's torn the retaining ring off. Material is far too hard & will not flex to slip into the retaining ring.
My advice to anyone contemplating re sealing an LGR is DON'T, it's bl**dy near impossible & I can see why Walther recommend fitting a whole new piston & seal assy.

It's possible that the seal supplied just isn't up to the job (red in colour, walther original is blue), but I can't very well send it back now I've ruined it can I.

I'm mechanically very competent & have re sealed & re sprung numerous air rifles in the past, but this one has just about beaten me - Can anyone recommend an airgun repairer in Essex that will take on the job?

Wish I'd never seen the blessed thing in the first place now