Thank you for your reply, Peter
I've got an external Flarm antenna for the P3i side of things but the PAW itself will be mounted underneath and to the rear of my seats, close to the aluminium box, that is the luggage bay. In order to remove the ADSB antenna from the shielding effects of all that metal I thought I would remote mount it above and behind the box, so that it has almost 360 deg coverage. If the end-fed dipole is going to be as good as a centre-fed bit of kit then that is great, as I already have the bits. I'll just attach the antenna to the Flight Aware dongle, mount that combination away from the aluminium box and then run a 3m male-to-male USB cable to the PAW.
I've got a spare 'Ted' antenna and would like to mount it externally for ADSB in but that would have to be relatively close to my transponder antenna and, as you point out, that's not a good thing. I
may be able to squeeze about 80cm between the two but I don't know if that would be enough.