Any idea what this thing in my boot is??

Looks like this guy may have cracked it

It is a second tank switched by that button on the dash? Did you try it?

I haven't had much time to investigate of late, the switch does not seem to be working but there are a few suspect wires hanging around. The contents of the drum was a yellow liquid, probably veg oil, but it had a very strong smell as though there has been some kind of solvent added to keep it from hardening in the glorious British winter

Ah, that could be waste veg with methanol or similar bunged in. JohnnyCrash is the man for sticking mysterious mixes through his fuel pump.

For the record I bottled it and only dino-diesel goes through mine. I've watched Johnny's various rebuilds with interest though.

bit unusual if they used the small tank for veg. but whatever. it's probably just a veg+petrol/diesel mix to prevent waxing