Right thermalmike i have found out what the problem is here and its a pretty simple one, take note thermaltake designers.
The blade is just too heavy and tight for the tube itself, when this type of plastic changes in temperature (with water) the plastic expands enough to make it stop spinning completely.
This is a simple fix for anyone out there with this problem:
1) use a fine nail file and sand 0.5mm from each edge of the blade
2) thin down the blade very very slightly again with the nail file/fine sandpaper, this will make the blade spin much faster at slower speeds.
3) dont shave too much off or it will rattle, you only want 0.5mm shaved off each side, before fitting back in the tube make sure there are no sharp edges.
Now i have done this, on my neochanger400ML pumps lowest setting (1500rpm), its spinning like a barbers pole and really quick/smooth.
So the problem isnt "really" with the design, its the blade, its too big/thick, thermaltake engineers havent accounted for the coefficient effect of plastics.
Im happy now its spinning really fast on minimum pump speed so its all good.
I like the design of this i really do