Hi Simbo , its easy , if you look in my picture ....... to the left of the DI is the white nylon T junction , coming out of the top is a metal tap , you set the tap to where you want , this determines how much water goes into your DI and how much goes back to tank , that is it .
If you cant source the parts call Vari Tech 01752 263333 , tell them Rich from PANES AND FRAMES has the return system you want and they will sort you out .
Rich P @ F