We still use the mk1's, have quite a few collars, and boxes, (on our last working box though).
Never used the mk 3 or seen it used, they seem to be a decent bit of kit, but a bit of a sting. If Deben had made them so the old collar's worked with the new box, it would have been less of a sting for the changover, it makes you reluctant to buy them to be honest.
I'm sure an electronics whizz could run up some new internal boards to fit straight in the old boxes.
Even my old feller can use the mk1, allthough trying to locate a ferret a foot down he'll still have the dial switched up to 8ft!