Thanks for the comments. My main concern was about the isolation of DCM inside the VFD but overnight it occurred to me that if I just check volts between FOR and DCM (expecting 12-24v or so), then both to control box ground, I can get some idea whether or not they are isolated. And if all else fails, I stick a relay in there. I've just found that I need to get a 16-pin IDC connector to pick up the analogue output on the UC300 as they don't provide that as part of the kit so things are going to be a little delayed anyway.

I have a copy of the manual - it's good that they show a circuit diagram of the board's interfaces. Gives a bit more insight into what's going on under the covers!