You may not have seen it before, but the cheapest place I've found for 'Daren Drums' (originally on ukcaving some years ago) is Key online:
http://www.key.co.uk/wide-neck-kegs_338462002010264760.html
Currently doing the 15L Curtec for ?17.50, free delivery, and the 3L and 6L 'caving' Daren drums for ?6.80 and ?9.60 respectively.
Once Key have your address they send you lots and lots of catalogues, and the occasional 50% off / ?50 off a ?100 order voucher.
The Curtec drums aren't quite the originals, they're roughly the same size but have a much coarser thread than the original 'Daren Drums' which I believe were made by Merck. I don't think they're as waterproof as you can't apply the same force to the gasket, and I've also seen a couple of fractured lids.
I think my favourite non-going-underground Daren drum size is the 10L one. It's super stable as it's wider than tall, and is just the right size + accessibility (as nothing can be buried too deep and you can get both hands in there for a fettle) for a base camp 1st aid kit.