Rear shock confusion

[The Teutonic Tangerine]

I may be wrong but it looks like that nut could be a lock nut for an overall length adjustment, rather than preload.
You may need to disconnect the shock to wind down the bottom yoke after releasing the locknut.

I would suggest that this is a non bike specific after market shock rather than an OE.

[Jeff Highland]

That's what I recon too.
But I suspect that, unless the spring is too soft, an extra 10mm or so length if available on the shock, magnified by the lever arm, may resolve the issue.

[fontana]

You are both correct.
I've finally got to the bottom of it thanks to a visit to a very knowledgeable local mechanic I know.
The rear shock is indeed an aftermarket item, significantly shorter than stock, fitted because the last owner must have been very short and struggled with the ride height.
I noticed when I picked it up after buying it, that the saddle was on the lowest of it's three settings, which clearly still wasn't low enough for him so he fitted the shorter aftermarket shock in order to lower the back end.
That's why my centre stand grounds at moderate lean angles with a passenger on board, and also explains why my bike sit's almost upright on it's side stand.
So whilst a bit of a nuisance, it is at least a nice easy fix for the issue.