I have tested again this morning, when I replay the track I can see LHR and Gatwick traffic. I dont know if I was zoomed in too far previously, or I can now see traffic in SD because I have all the dongles connected.
Anyhow, is this to be expected.. I know there was non-commercial activity around fairoaks, and would expect movements at biggin and farnborough, even though it was early on Saturday.. ADSB warnings at least?