Firstly he checked both pre (297ppm) and post (22ppm) treated water using his TDS meter. It did give rather different readings to my eBay jobbie but still proved what I had been telling him. He then used a calculation to work out te number of litres of pure water I should get out of a full DI vessel with new resin in. It worked out to about 750litres of pure water. He then said you probably use about 30 litres per rinse for a normal car but the X5 is a lot bigger, but using 30litres as a guide gives about 25 washes. So with the X5 being much bigger that may end up being 16ish for example so possibly about right for the number of uses.
Still at this rate it's potentially going to work out quite pricey (and I totally hold pvr responsible....

He did advise that another option is to add an RO system (reduce my ppm down to 15-20ppm) in place to fill a water butt to use the DI off that as the RO will apparently last a couple of years of use and the DI too would last a number of years. But of course that would cost me...lol.
Hmmmmmmmmmmmmm what to do, what to do.