The belt has almost an inch or so of play in it and looks perilously close to skipping on the teeth. The top section is about as tight as the other belts are.
Following a nice chat with HAC, I've actually now rejected the printer and as agreed with them, it's being returned to be swapped out with another Ultimaker2.
As, I discovered that when the printer sits on a flat surface it will rock on two legs so something somewhere isn't good! The shipping box has signs of impact damage on one corner (which I didn't spot) so it would appear it may have become damaged in transit.
Shame, these things happen I guess, but I reckon in the meantime I'll just spend more time getting to grips with OpenSCAD, as I have several more projects in mind now!