I had been looking at this one for a while until I managed to get a second hand 200L for £100. Have you tried looking on Gumtree or your local second hand ads?
I'd go for Gumtree or local advertisements. I saw a 300L tank with supplies and stand for 200 pounds, but it was a cube tank. I've also seen just 100L tanks for 5 pounds on local advertisements earlier today.
Yeah I've been thinking of gumtree, ebay etc, nothing quite fits the bill as yet. I also worry a bit as to how well its been kept, if it's seals are still strong etc..