Or you could just buy this maybe http://gardinerpolesystems.co.uk/all-products/pump-hose/pump-controllers/system-controllers/liquid-logic-trade-autofill.html
I brought a 12v version of this as I was going to set it up in my van but I changed my mind anyway it is new and in the box cost around £300 if you are interested you can have it half price as it is just gather dust http://gardinerpolesystems.co.uk/all-products/pump-hose/pump-controllers/system-controllers/liquid-logic-trade-s2-static-ro-system-booster-pump-control.html