Update

Well, the HW35 didn't take long to blot it's copy book: The piston seal failed after approx. 50 shots.

The rifle suddenly became very slammy and a strip down revealed that the piston could easily be pushed down the cylinder with a finger over the transfer port.

On closer inspection there appears to be a small deformation on the edge of the seal. It's not burnt or damaged and appears to be a manufacturing defect.

Fortunately, I had a spare piston seal from another project in my box of bits. Fitting this resulted in normal, smooth service being resumed.

Unfortunately, the rifle then turned out to be making an easy (and very smooth) 14fpe!

The spares box again came to the rescue with an old style HW80 spring. This had the same dimensions, wire gauge etc. as the the standard HW35 mainspring apart from being considerably longer. I chopped it down until it was 2 coils shorter than the standard item (and still with about 15mm of preload).

This is currently giving a lovely smooth 11.8 fpe and very easy cocking. It's a little close to the limit for my liking, but I'll leave it as is for a while a see where it settles.

And, as an aside, HW build quality really does seem to have gone to rats these days.