FINALLY got the battery tray out, done the linkage reset, and failed miserably to replace the reverse light switch (don't have a deep enough socket and not enough room to get a spanner in, so it can wait till service time next week). However, on the bottom of my battery tray there's a kind of rubber pipe, and I can't see where it's come from or what it does? Where am I supposed to reattach it? I wasn't expecting anything to be connected once I finally got the tray unbolted so I wasn't watching where it came from :(

I don't think it goes anywhere and is just a water drain pipe?

Ah, you could be right. I just looked and where it attaches there's a small drain hole. I'll just poke it down the inside of the arch then. Cheers! Thought I'd made a proper hash of what should be a simple job then!

could be a vent pipe for the battery

Vent pipe for what though? I thought batteries were totally sealed units these days?

its a water pipe to get rid of any water that sits in the tray.