Tacx Flow Smart trainer calibration issues with 700x35c tyre
After listening to advice from @Carel, I warmed the tyres up for 20 minutes, a lot more than I would have envisaged, and I finally got consistent results being able to calibrate more than once.

After a bit of slackening, I managed to calibrate to the centre of the green zone. I can't wait to re-do the whole process after my training tyre arrives!

A quick thank you to @AndrewHenle and @DanielRHicks for their time and help also :-)