@ Jon D
glad to see that you solved your problem
I like your object orientated way of programming, using classes
My bot is currently belly up, I will try your code as soon as I can.
Not sure if the words "FEZ" and "Domino" are allowed on this site 
@ Onions
Hi welcome on the right thread
where you got the wheels? (A UK supplier would be nice!)
I got my wheels here
I am not familiar with UK suppliers (only this one , submitted by GuzZzt)
Others may jump in with additional links