Remote Spring Preload Adjuster Trophy T3xx Series

While out on my 2002 trophy 1200 with full panniers, top box, tankbag and pillion, the handling was an issue as if the rear spring was not on the 5 (max) setting. Before setting off I dutifully set it to 5 (i.e. max preload), but on returning I checked it was on 5 and then wound the preload out to 1 and it didn't seem any different to 5, no lower/taller, no difference could be felt with respect to how easy it was for the feet to touch the ground. Went between 5 and 1 a few times to see if it would loosen anything up but no luck. So I'm coming to the conclusion that the preload adjuster is at fault.
So my question, is the spring preload adjuster serviceable?
I've never needed to look at the rear suspension on any of my T3xx models that I've owned since 1992 so have no experience on how the internals work. I assume it's hydraulic, perhaps a cylinder that pushes down the spring as the adjuster is increased?
Anyone experienced this and found a solution without swapping it out for new suspension?
Thanks.