Reporting back, it appears that the problem has now been solved. I took the Longbow back to the gunsmith, who sold it to me, and then stripped it and re-greased it (using Bisley gun grease, I think it was). The rifle is now performing well and the power is up nearly 1ft/lb, depending upon the pellet, and is doing about 11.5ft/lbs. Air Arms does best and VMX is about 10.9. Four different pellets were tested.

The gunsmith (Westons in Brighton - very helpful people) was of the view that the rifle had dried out, after some 14 years, perhaps.

The instruction manual does advise lubrication, periodically, so perhaps this is the lesson to be had from this - probably, so long as it is not overdone.