petsi I have a twin 4040 on a booster pump, stainless steel housings, all professionally plumbed with John Guest type fittings but they are nickel plated - very cool.
I know its way over £500 but the 4040 route is the wa to go. I think June at Gapswater does a 4040 kit for about £500 but if you go 4040 get the HF5 membrane.
FYI - my system runs between 600ltrs/hr in cold weather to 1000lts/hr in the summer - I can run it quicker but the guy who built our systems recommended running at 100psi and 50/50 - system is about 4 years old and still produces water about 006ppm from membranes - we use approx 4000 ltrs a week.