Very sensible. :D I've just ordered a pukka Arduino R3 Motor Shield. My knock-off Mega tests AOK using Gregg's advice, so I shall wait until...
Thank you @Atani , looks like you caught my earlier edit. Yes, I understand what you've suggested, and it matches the Arduino Motor Shield......
How far have you got? :) I've just received a pair of the FunduMoto motor shields to go with some cheap MEGA2560s. I do want the current sense...