Thanks for the responses.
Yep it's definitely M32 X 1mm. It snapped while I was undoing it (to my surprise; it was stiff but was unscrewing without much resistance). I jumped on eBay and quickly found plenty of m32 nuts with a 1.5 pitch but figured I'd just check with a thread pitch guage before ordering, which is when I realised I should have left it to soak in wd40 before going at it
I can find a couple of new old stock items from the usual European suppliers (which confirms the sizing), but since I've just put in a big order with ost2rad and had no plans to spend a silly amount on postage for a single nut, as well as Brexit now being a thing which I'm not sure as to whether that will sting even more on postage/import costs, I figured I could just put the question out there incase on the very rare chance that someone had a spare floating around that they would be willing to sell me.
I'm currently in the process of refurbing the electrics so if I'm lucky I'll discover a few more bits that need ordering which will at least slightly ease the blow on potential postage/import costs from the eu