Gary, I have one of these coil over shocks fitted to my Series D Comet (replica), and although I did have to remove the seat and tank to initially fit the shock, you should not have to do any of that to change the preload or the damping settings, the preload should be accessible by lifting the seat up and using a C spanner (rather annoyingly sold separately) to adjust the threaded collar to compress the spring, the damping setting is just an Allen headed adjuster near the base of the unit, clockwise for more damping, anti clock for less.
I have to say it is a pretty fine balance to get it correct, I normally ride two up and in the end I requested a heavier spring and used it with very little preload, and I think with the damping on the max setting, works fine now, but can bottom out occasionally when working really hard, appalling roads etc. It is definitely superior to the standard Armstrong unit and also the Spax unit that came fitted to my bike, I would imagine the Works Performance one would have been a good one to have, sadly no longer available. We are lucky that AVO even bother, I doubt that it has even paid for itself yet, it probably never will.