Darren,
Just a guess........sounds like either the pump, or, the control unit.
What I would do is:
Put it on a bench or strong table (so as you can work on it )
Take out the battery pack.
Remove the pump cover (black cover screwed on below the battery bay)
Attach one of your small poles.
Then get a couple of wires from the battery BE CAREFUL NOT TO SHORT IT!!
And run them straight to the pump. Eliminating the controller. Attach the Negative to the pump (if you can) and touch the live....see if the pump runs top notch.... if so it must be the controller, if not it's the pump. Either way it's a £60 + job to fix it?
Hope this is some help.
David