Getting the thing installed was a bit of a puzzle as it has many dependencies (and that's just the compiler) but I now have it working like a dream on both my laptop and my netbook. I discovered that it didn't work with my presenter mouse but with a bit of help from a friend, I have a patch for that and now when I'm presenting I see something like this:
You can set which screen show this, and which shows just the main slide, and you can also set what duration the countdown timer should start from. One really key feature is that the timer doesn't start counting until you advance from the first slide ... unlike in open office where I usually put up the title slide during the break before my talk, then have to stop and start the presentation to reset the clock so I've got some vague idea of my running time!
So in true open source form, there's a tool out there already (thanks Jakob, and thanks for responding to my emails!), and I was able to adapt it to my use case, or rather Kevin was able to! I would love to have the presenter console packaged so I could recommend it for more users, but for now I have a great open source solution enabling me to do what I'm good at - delivering content.