Thanks for the advice guys. I will probably go for the pumped system as the difference in the fill rate is amazing.
I presume if it only takes 30 minutes to fill a 500 tank don't really need a shut off solenoid as easy to monitor. At the mo I have a ballcock in the tank which cuts the merlin when full, saying that for some reason the waste water still flows!!!
I very much doubt your gonna fill a 500 lt tank in half and hour with 1 40-40 membrane and a booster pump, you need to be making about 16lt a min of pure alone not taking into account your waste ratio.
You can run a 40-40 at any waste ratio you like but you need to find the balance of making the pure water and not overworking your membrane to make it last and get the best out of it.
At 320ppm from the tap you definitely dont really want to be going any lower then 50/50 if it was me i would be running it at 60/40 waste to pure.
My tds averages at about 570-590 and i run a bit less than 70/30 my membrane has been running for 15 months now and it still produces 14ppm before di vessel (the same as when i first installed it) making about 500lt-600lt of pure a day( i am not on a water meter).
Ideally you also need to check your tap flow rate first when using a booster pump as most need a minimum flow to work properly, having said that most mains flow are good enough but best to check first to be on the safe side.