Just to draw a line under the original problem, in case someone searches for the same snag.
I fitted a Flight Aware Pro Stick between the ADSB antenna and PAW as my ADSB dongle. What is very strange is that it seemed to work very well at home and I picked up a gazillion contacts (pre-lock down). I was using the official Raspberry Pi power source, so had loads of Amps wiggling down the tube. When I installed the same set up in the aircraft I got the Red blob on 1090 and, despite changing the Pro Stick for another one, got the same result. This was using my Charge4 with 18AWG power cable supply to the PAW. Engine running and not running made no difference.
Enter Ashley The Saviour
He saw a snag with some of the gibberish in the logging page and said it was my ADSB Pro Stick dongle that was the culprit. A bit strange, as it definitely worked at home but I'm not going to argue as I don't have a clue about those log codes (parents, babies, $$$....who knows). Luckily I had an SMA to MMX adapter and Ashley sent me a new Nooelec dongle, the same type used in the Rosetta. Apparently these use less power, so that all sounds good to me.
The result: a green blob most of the time BUT an orange blob quite a lot of the time as well. I was concerned about the orange blob as I didn't want to put the seat pan etc etc etc back in and then have to take it all apart again. Another call to Ashley revealed the 1090 orange blob isn't a problem. Basically this means your ADSB has seen traffic in the past (green blob) but it's not seeing anything at the moment. Red is not good but orange really means it's just waiting. With the lack of traffic around at the moment that is not surprising. Relieved that I hadn't done something wrong the machine was re-assembled and fingers double crossed.
I flew yesterday and, according to the traffic log, I am pleased to note that my PAW saw many, many things. I also saw quite a lot of traffic (especially to the north of Compton Abbas). I saw PAW and I saw ADSB and the lovely lady even warned me of the danger presented by the aircraft ahead of me on final
So, if you get a red blob and 1090 unavailable, change your Flight Aware Pro Stick for something else