Thanks Evil. Good to know one of these things works ok for someone!
I raised instantaneous consumption only because it is just so whacked on my Ultragauge that I suspect that's the problem. The distances actually look pretty right - give or take a % or so. But the economy is almost exactly twice what it should be and twice what I can calculate it to manually.
I did try different settings and so on around fuel type / tank size / cur off etc. None of it seems to make any difference at all.
Everything else on it works great - except the one thing I bought it for....