Technophobic ran Hydraulic drive with a small 2 stroke petrol engine I believe, this was back in series 2 and 3.

Might be stating the obvious but you can only put down so much power when your robot is limited to 110kg. You would have to look at magnets or maybe EDF's to pull it to the floor to make use of all the torque and power.

But it would be great though to see an all hydraulic machine again!