Update for anyone reading this in the future:
Removed the rear interior plastic cover to then remove the boot lining on both sides. Removed all flooring, tools, tyre etc.
Made sure everything was completely dry and shoved bits of paper towel anywhere that I suspected may be the entry point (e.g. under rear light fittings, under boot breathing vent, around boot lid arm etc).
Was unsure if the leak would take time to seep in and wanted to keep track of where the water was coming from so just did a few drops of different food colouring in jugs of water and poured them in different places (e.g. red directly into the right hand drainage channel, green down the sides of the roof and rear windshield, blue over the spoiler connections etc). Worked perfectly: the only colour to show up indicated it's only water flowing straight down the right side channel beside the boot seal.
I believe it's normal for that water to flow down that channel, and a lot of it to pour through the gap of the rear light fitting and down through the light fitting to drain underneath (which is what occurs without leaks on the other side). In this case the water is hitting the connection for the rear parking sensors and running inside. I assume the rubber seal is broken or not seated correctly, I haven't had a chance to look yet.
Easy fix, I'll pull out the connection and either replace it, bodge with silicone and foam, and if it still doesn't work I might even just use some of that black gooey sealant stuff around the seal. I also imagine the inside of the light casing may need cleaning if the water hasn't been flowing straight through like it's meant to.
So yeah: removing the linings etc is pretty straight forward. I wouldn't be afraid to give it a go and if you've got an unknown source of water, the paper towel and coloured water trick worked a charm.
