You have switched it on at the key-switch on the battery, and the battery is fully engaged in its docking station?
Assuming so, you can't do anything without a voltmeter (multimeter). First you have to measure the voltage between the outer two slots in the battery (after removing it). That should show about 28v. If that's OK, you have to disassemble the compartment under the battery. You only have to remove one side to get at the controller connections, so remove the side without the key-switch (two screws in the side plus about 3 small screws in the corners). With the battery switched on, check that you have the same voltage on the two black and red power cables going to the controller. There'll be bullet connections that you can disconnect. Report back when you've done these things, otherwise take it back to Tesco.