All sorted. R/H drive motor was wired incorrectly internally. Swapped the cables at the controller +/-.
After laying in bed thinking about it, it made sense as the drive was slow (steering speed) and the turning was fast (drive speed).

Well there is still a steering pot and a traction pot. Could their connectors have been switched? It's been awhile since I've had trouble with one so I don't remember the connector type but it wouldn't hurt to check. if you put your email in your profile I might have a wiring diagram I can send you