Hi there - this is the dongle -
https://www.rtl-sdr.com/tag/low-power/ the description looks like it says the signal is decoded on the Pi so probably a software thing?
I cut off the old aerial and created one from the remaining cable. I then used the dongle that came with the unit originally. When I do this, within a minute I see a couple of aircraft as per the attached. Looks like the dongle and unit are okay, and it's the aerial that I bought that seems faulty. If I can find a soldering iron, I will fix the one I broke properly but I'm away for a couple of weeks so I'll need to see about it when I get back.
When I plug the new aerial in, the number of planes drops and they 'come and go', switching back to my bodged aerial leads to an increase back to what you see attached.
Does this look right to you? Thanks very much for all the help!
Cheers
Francois