The project came about after finding a source of wheels to fit widely available geared units. After a little experimenting, the L239D was found to be happy powering two pairs of geared motors (two motors in parallel per channel).
A 14m PICAXE microcontroller will give you 2 spare outputs and 5 inputs whilst controlling the motors.
PICAXE microcontrollers, programming software and information can be found at http://www.picaxe.com/ with a very helpful active forum.
Kitroniks have excellent wide wheels for geared motors. Wheel for Geared Hobby Motor
The body for the robot was laser cut from 3mm acrylic, here is the drawing in 2D Design
An updated circuit board drawn in PCB design and make