Thanks for all the info lads.

I did a bit of work on it today, following the tuning guide (posted by Alistair IIRC).

Degreased the internals and the piston (the grease in it had near on turned to a powder), sanded out the scratches on the piston, ready to be polished, and am now waiting on some dri-slide stuff to coat the insides before I can continue.

Got a new spring, piston head, buffer washer, piston seal and breech seal for it so hopefully, I can bring it back up to par.

The Mercury, even though it's long been left behind by modern springers, is one of my favourite guns.