Spoke to Mark who made this video and he said for me this is the way to go. I only need 100 litres a week on Saturdays. And for me this is better than an RO. I think the principle behind it is pump the tap water into the first DI which is a bit rundown (meaning the TDS coming out is 16 or something) then put it through the next DI which has fresher resin and this brings the TDS down to 0. Is this right? Anyone done this? He reckons can make 4000 litres for £70. I live in Bournemouth and TDS is high (over 300 think), but this should still work here apparently.