I'll probably do that eventually but in the meantime could I wire in a switch on the handlebar to switch off/on the motor without having to use the key on the move. I'm thinking a switch across a brake switch but would appreciate any advice/suggestions.
No. that wouldn't work. You need to switch off the controller. the brake switch only stops it from outputing power. Some controllers have "ignition switch wires", which you see as a pair of wires on a two pin connector, often orange and red or pink and red. A bridge is connected to short the wires, which switches it on. If you have that, you can connect a switch instead of the bridge.
If you're unsure about that, pull out the controller from the compartment under the battery and spread all the wires and connectors so that we can see every wire, then photograph it and post here. You can't put a switch on the battery wires other than the switch you already have.