Thanks all for the help. Tiger - Yes, it is def wired up right. There isn't very much for me to get wrong. I agree that the diagram is rubbish.I do hear the big relay clicking in and out every five seconds or so, in time with the controller LED showing connected and disconnected so I know it is really doing it.
Eds - Alas, this is too late for me :-/
Dave W. That was also my thought. Either Hysteresis or voltage thresholds with a timer. I could probably build my own, even if I just use a manual switch to link and unlink - I might open up the T-Max box and see what is in there. If there is a pot I might try adjusting that to see if it is just thresholds. I suspect it will just be a pic or Atmel and I won't be able to change anything.
It may be that the two batteries are just different voltages. I see the aux battery at 12.8 and the primary just around 12. My concern is that leaving them connected isn't going to be healthy. I don't know so much about these big batteries, particularly Optima...
Steve