Ok, I ran the Magnum 350 with the polishing cartrdge over night for 12 hours and the water is now crystal clear. I did this on Friday, the water is still crystal clear.
So I deduce the cause to being a lack of rinsing the matrix prior to putting it into the HOB filters...
Thanks everyone for your help !
I am going to start to writing all this down - too many things that you have to remember !