The other drive motor appeared this morning, so I added it to the test bot. Here's a video with it with all four wheels:
http://www.youtube.com/watch?v=-9OCh4_EpBQ

I'm quite happy with the speed; its currently running on a 12V 3Ah botpack so weighs about 1.2kg. Hopefully the speed won't drop too much when I switch to a 11.1volt lithium. That should at least give me the weight for important things like a weapon and armour! I think I'll have room and weight for 4 cells but I'll wait until its finished and I know what the actually weight is before switching. Solidworks says that as modelled it weighs 1290grams, without wires, connectors or fasteners. I think wires and connectors should weigh about 30grams and the fasteners the same, so its going to be quite tight. I don't have any scales accurate enough at home to check if the components I've received weigh the same as their advertised weights. Fingers crossed!

Final picture of the bot as it is:

Hooray for duct tape! :-)