Ah thanks guys, I'd seen that guide before, but couldn't find it again online. The Haynes wasn't bad, as it goes.
It was pretty obvious to me now after pissing around a bit and studying both guides and the actual roof mechanism that I can't slot this rain tray in properly without actually dismantling the roof and taking out the whole assembly including the guides. What I've done is actually get the rain tray in securely by jamming the tabs underneath the rail and it sitting just on top of the rail behind the sunroof. I had to kind of slot those tabs in underneath when the roof was tilted. Took a couple of goes to get everything lined up. Makeshift, but I'm pretty sure it's serving its purpose.
I decide against attempting to go one step further and take the rails out because I noticed one of them is cracked and looking a bit delicate. I'm a bit wary of breaking anything further by doing this.
I've settled for having a non-operational sunroof (never use it anyway), however, the drain holes work, the rain tray sits in the correct position (albeit half a cm too high) and is secure, and the roof now sits nicely flush with the rest of the car roof instead of permanently being up on one corner - I adjusted the cables - they had got out of alignment. Also the whole thing doesn't rattle around any more in that annoying way. The important thing to me with this was to get everything sitting right, and address the water leakage, which I think was due mainly to the rain tray being at the back of the cavity and not in its proper position. Ending up in a brown water stained headliner which dripped water on my head.
Well, an improvement I guess, although it's non-operational, at least the water won't get in no more
Bit of a pain these things aren't they. They need very delicate operation, if they're forced at all, the cables slip, and you get that ominous cracking sound
If anyone knows some kind of David Blaine voodoo to get the tray in without removing the rails, please let me know, I may do it at some point if can be arsed. But I'm happy with it as it is, tbh.