If you get a bit of swarf behind one of the idler pulleys or don't to fit the correct thickness of washer between a pulley and the block then the cam belt will run out of alignment.
You can have the same problem if the lug on the water pump is in the wrong place which prevents the pump seating square to the block. If that happens it will normally widdle coolant all over the place before you get as far as starting the engine & throwing the cam belt.
I would have the tensioners off again to check that the correct washers are fitted behind each pulley & have another look at the water pump to check that the lug hasn't stopped it seating properly.
Rechecked everything and all was put together as it should be. I took off and measured the depth of the pulleys, tensioner and the water pump.
The brand new water pump, a CDWP02 from buypartsby is so far out of spec from the VX one it was forcing the belt over so far, that it was touching the engine block. I took the rubber seal from the new pump and stuck it on the old pump just to make sure it was the pump at fault. It is the new pump at fault.

The belt is now centring as it should do.
I will be buying mostly VX parts in the future!

Thanks Andy and thanks to everyone else for your help.
