So I went flying today with the replacement PAW supplied via Dave and Lee.
And...... EVERYTHING WORKED PERFECTLY!! Finally, I had fully functioning traffic, ADS-B out, no radio interference... and I even had my own registration appearing loud and clear on the traffic page, every time I looked - see attached pic.
I did also move my new PAW unit so that it is now around 50 cm from my radio; I do wonder if its previous extreme closeness was part of my serious noise interference problem.
On the balance of probabilities, it does also seem that there was a fault with my original unit. I know I had a dodgy aerial and ADS-B dongle, but it does seem that there was also a more fundamental problem with the underlying unit as not once did it show my own registration on the traffic page when connected via my TRT800H; the new unit did so immediately and continuously.
Now to get back to enjoying the flying!
Thanks guys, and in particular Lee, Peter and Dave who have been super helpful throughout. Dave - my old unit will be in tomorrow's return post to you.
Best
Stu