Ah yes, Tk::after seems to have sorted it. The "repeat" call as the one I was looking for. I had looked (very quickly, admittedly, and without much gusto to be honest...) for a class or function called "timer" in the Tk docs but the (IMHO) bizarre naming of Tk::after had led me to overlook it.

However, Tk::after now in place and all working well. I now have a series of bars going up and down for pretty pictures!! :)