thanks for all the responses. Tested again this morning with the same result but when I uploaded, it came back as positive. Called GP - 'not us, try 119' , tried that but after listening to about 20 different options I gave up and tried 111 who were much more helpful. A clinician rang me back within 30 mins and referred me to the Anti Viral team. Had a call from them and have arranged an infusion for today at 14:00. So the system does work, after a fashion, but you need to be persistent.
I will say that the GP surgery did ring me back to see how I got on which was unexpected but appreciated.