Bit more messing around with the one. So the neck itself definitely flexs if I brace one hand on the heel end and push back on the headstock. I know that this is generally possible with all guitars, but this one does feel a bit softer than you'd expect. I'm not sure that's the whole story tho as if I brace the guitar body and push back on the head stock I can see the whole area here flexing:



The joint it'self looks solid, there's no cracking or anything from the inside and the cracks around the heel on the finish don't appear to move or anything. The finish where the fretboard touches the body is also in good condition with no cracking so I don't think it's moving at the join as such, rather that whole area is flexing.

I've been searching around and came across a few stories of SG's moving as Simon mentioned above, but nothing so far like this.

I think it's either always been like that, or it getting hot at some point has somehow softened the wood. It's pretty odd.