After a busy couple of months spent moving house, doing renovations and re-arranging work I've finally got my BigTrak project back out. I tested a circuit to allow me to re-use the existing optical encoders, burnt a couple of transistors testing out a H-bridge before settling for an off the shelf motor driver from ebay! (I guess bc237/bc238 transistor combo's just aren't cut out for driving motors but theyre the beefiest I had to hand... )