So, some good news.
I loosened the jubilee clip and twisted the potentiometer sleeve on the shaft and felt what I can only describe as a pop. Afterwards the sleeve moved freely between 2 points within the fixed part of the potentiometer so I decided to turn the wheel on and test it manually. When I turned it on the wheel did not start spinning right away as it had before, my initial thought was that I had broken it. But to my surprise when I turned the sleeve the wheel started and got faster the more I turned. I could turn it then the other way and it came to a complete stop again.
Encouraged by this I twisted the sleeve so the wheel was not spinning, made sure the pedal was as far back as it would go and then tightened the jubilee clip to secure the sleeve in this 'off' position. I then tested it several times with the pedal and the wheel stops completely and the speed can be controlled quite well.
It would seem that the sleeve just wasn't in quite the right position.
Thank you Bill for the advice on where to look and what to do. It seems I have gotten it working for now.
I appreciate what others have said about it being a cheap Chinese wheel. I think you're right, but with the seller not accepting returns I think it am stuck with it. It wasn't expensive and will hopefully be a good starting point for me to perfect my throwing technique so I can move onto something better in the future.
Thank you to those that responded. If anyone wants more information or detail on what I did (not that there's much more to add) please just ask. And if anyone is interested in how I get on with the wheel I can always post something here.
Neil is correct, interesting machine. First step is to confirm it stops when the shaft potentiometer is at its minimum position. The potentiometer appears to be the black tubular piece that has the wire attached to it. This is just like a volume control knob. Likely three wires in the cable.
It appears that the most likely thing to check would be the stops and another calibration possibility would be the hose clamp that fastens this assembly to the stop shaft.
So what would I try? I would loosen the hose clamp and confirm (gently) that the potentiometer minimum and maximum positions are indeed reached and at minimum travel the wheel is still moving. If still moving then we have to find an electrical limit.
Keep in mind the potentiometer is not made to take a lot of force so the mechanical stops absolutely need to be set correctly and limit the force on the potentiometer
From there I need a picture of the control board and a bit of luck to spot where that cable from the potentiometer plugs in to take an educated guess at the electrical limit adjustment if there is one.
No guarantees, all would be supposition but these things often are somewhat identifiable by sight. I am not sure the control board is visible though, it may be fully enclosed.
Not sure that helps but more pictures especially where the pedal speed control wire terminates at the circuit board could get you further.