I disassembled the gun again, and with 90BAR to discharge from the cylinder, I manually used the hammer to discharge air. (Don't do this at home folks!) Heavier blows released more air, so I ascertained that a heaver spring would give the required result.
I have replaced the spring, and now get 11.2 to 11.5 ft/lb output!

This may or may not be a long term fix, I will put a tin of pellets though the gun and check for consistency.

The original spring looks and feels fine, so I am at a loss to understand the sudden loss of power. I found a thin layer of oil in the cylinder, but did not dismantle the air valve or access the firing pot.

Thanks to you all for your suggestions, I will visit BTAS next time I have a free half hour