Quite correct Chrisj. Wiring diagram wasn't needed after all.
Rather than pull the bike apart from the lights, I started at the battery where the main feed to the relays begin - and there lay my problem. The fuse holder had melted along with one leg of the fuse.
I dug out a spare fuse holder, connected everything back together with a new fuse, and I now have extra lights again.
Double checking myself this time, I checked online what fuse rating should be running those lights. Turns out that 15amp is more than capable. The fuse that came with the lamps was a 30amp!! And numpty me

(at the time) just used what was in the box instead of checking first.
Sometime in the past 8 - 9 years of running, something must have gone awry (probably too much rain one day) and the fuse should have blown then - but being a 30amp it just carried the current regardless.
As it turned out, I didn't have as much time as I thought I had to troubleshoot, as seeing as the new fuse holder and fuse is working, I'll leave it be for now. At least if a problem happens now, the lower fuse should blow instead of melting things.
Oh, and thanks Houdini, I found what I was looking for in your link.
