Thank you very much Gents, your comments are appreciated, as I 'aim' to please (did you see what I did there ??? )
The Vulcan is a lot more evenly balanced than the BSA Mercury, and so imho, is far more pick-up-and-pointable, and on average, the Vulcan was 1.5ft.lb more powerful (my experience was that back in the day, the Mercury struggled to cross the 10ft.lb mark straight from the box).
I also prefer the open sights that came as standard on the Webley Vulcan over the Mercury, too, but BSA had it pegged with the trigger, but only just though, as both were/are inferior to the HW35 triggers of the day (this was pre HW80 and HW77).
Notwithstanding the roll pin in place of a breech bolt as moaned about in my opening post, here, however, for me, the Vulcan was the complete package, as being a late teen at the time of its launch as I was, the visual aesthetics of the Vulcan gave it much more sex appeal than the other two, with only the later FWB 127 and 124 offering more trouser movement.