thanks lads, never thought of doing it the other way with the di after, is there any way i can keep the auto switch off do you know?
You can fit a shut off valve (small ball valve) to your tank. Mine has one on the hoselok inlet (filling) port. 
it has one fitted now but its to the RO system, can i just adapt it to the DI system? or just put another type of valve on there?
Not sure what you mean mate but a small float valve in the tank is what I use on the back of a hoselok male connector.