I use valet Pro car wash. A little goes a long way with them. Comes in 5 litre containers but I'd say, £ for £ it is the best stuff I've used and works out cheaper than the cheaper high street brands.
Another good one I use is the Tesco's own range of eco friendly stuff.
Was about £2.99 for a litre, works well and smells nice too and kept the Enviroment Agency happy when they did a visit on my old place and was asking what went down the drains!

They were well impressed when they saw me using the Tesco's enviro stuff!!
Hid the hardcore stash from their prying eyes!