Despite looking for a while I can't find any answers, which either means I'm doing something very stupid and obvious, or - well, I don't know. It's probably that.
Running Lubuntu 14.04 on (sadly) an Asus eee x101ch, xcompmgr works fine making Cairo Dock transparent. I've been reading about Compton, and thought I would give it a go - but it doesn't seem to make Cairo transparent. The best it does is turn the black panel the same colour as the desktop - but it's certainly not transparent, as it hides other windows.
I only installed this OS a couple of days ago so in all other respects it's "as new".
Any ideas?