Yep, the front wheel is the tensioner, it does its job in keeping tension but not much in keeping the track on...

been doing some reading on flat belts etc and the strange thing about them is that they are supposed to be tolerant to a misalignment, cant understand how as in my case its completely opposite.

This afternoon had a go in driving it with just one track but just after a cople of seconds the track was off.
Very frustrating after all that work... will have to do some more research and a major redesign. ...if not scrapping the tracked design all together