So are ply lining and racking a mod for plummers and the like
Ply lining? That's a very good question.
It's fitted after manufacture, so it should be a modification. It reminded me of the fitter who fitted ply lining kits to 900 Berlingo vans ordered for a major fleet order back in 2001/2 and supplied out of a Citroen dealership in Sheffield. The fitter used the wrong length of screws and puncturered 900 fuel tanks. That was definitely a modification of 900 fuel tanks.
Racking is a modification, as is a roof rack and tow-bar. Insurance company (Aviva) specifically asked me that and advised they were modifications.
My water tank is also a modification. I was asked if it was professionally fitted. I said I fitted it myself and was professionally fitted. That was good enough for the lady asking the questions, but I would expect it to be queried if I made a claim.
Interestingly, I fitted the tow bar and roof rack myself as well, but they never asked if those were professionally fitted. I've put that thought in my 'ammo-box' if I ever need it in the future.