Hi
The advert prices a 300gallon per day RO unit with 3 membranes for £131.00. Thats about half the price of a similar one from RoMan etc. 3 decent 100 gallon membranes aren't far short of that.
Your restrictor valve will be on the pipe to waste and will be near your flush valve. When you open the flush valve waste water by-passes the restrictor.
Spruce