Glyn
i agree with you, plenty go cheap, i know i did, built a trolley system using a trolley the same as Peter f's a pump box and a RO unit that does 100 Gal's a day
total cost 625 quid, compared to the more expensive "off the self deals" its about half price
you backpack and cart i guess will knock the price down a fair amount from what i paid (i know it will as i know the price of the backpack and very good it is too
)
couldnt you have 2 types of trolley/cart to hold the backpack, the type you have in the picture and a more expensive option, with closed sides and a lid on it, it would hide all the "workings" and look better