I've finally managed to solve my airspy_rx / rx_tools conundrum.
After a couple of days of getting nowhere, decided to go back to airspy_rx and use csdr functions on the command line and twiddled about until I got a good terrestrial FM signal (yes I removed the bandpass filter first..)
Changed the frequency back to NOAA 15/18/19 and there was the familiar old 2.4 kHz carrier wave loud and clear. Got some good images too. Now just playing around with different decimations/filters and such like to tweak it. Possible stop using the -g switch and start tinkering with the three individual gains if someone has any tips I'd appreciate it...
Anyway, to clear, everything is now done on the raspberry pi 4. Not using spyserver at all so the main PC is free for rest of the family.
Here's my best pic to date: