I'm pretty sure that it's an antenna / cable problem. I've enough gain to see the PAW on my desk, but very little more.
I tried an alternative (in loft, but that's not good for open sky) antenna, and that seemed to pick up someone who flew right over the top. I've tried a different bridge to no avail - I've just watched someone fly past fairly close without seeing them.
Next stop is the connector in the loft - after that, it's swap out the antenna on the roof.
Thanks for the suggestions.
Paul