Final post from me on this subject....
I've flown a couple more times over the past few days.
30 minutes last Monday yielded 43 tracks. 50 minutes today 63.
Last Monday I got a traffic alert on an airliner 4km/2000' above - he was inside CAS, I was outside.
Today I got a traffic alert on another Cumbernauld-based light aircraft.
I have now seen and been alerted to traffic on both 1090MHz and 870Mhz.
I am delighted with the antenna's sensitivity. It is exceeding my expectations.
Incidentally, I believe that it is two half-wave dipoles mounted in a common body. Although it is mounted on a metal panel, there is no physical connection to it (unlike, say a TED) so it is not a quarter-wave unipole.
As I taxied in today, I received another traffic alert - '8 o'clock, same level, 240 metres'. The icon appeared on the apron at Cumbernauld, labelled 'Undefined'. After I shut down I went to investigate. It may have been another light aircraft moving away from the fuel pump. Looking through the .TRK file, the only candidate ICAO address is C13C3E which is, well.... undefined!
I am enjoying the post-flight 'plane-spotting', but I expect the novelty will soon wear off. ;-D