I've had a look at those speed controllers and they look a fair bit better and safer than the one I was initially looking at building.

Just a thought, would doing what someone has done for the steering on his R/C lawnmower (taken an ordinary servo, used it to drive an H-Bridge and used that to drive a motor with mechanical feedback to the potentiometer on the servo board) work as a speed controller if I were to remove the mechanical feedback and the centre the potentiometer/ replace it with a pair of resistors to effectively make it a modified servo?

http://130.94.182.150/mowers.htm - the link to above R/C lawnmower.

The making of the giant servo is under converting standard R/C servos into monster servos