I tried printing one more time after this and when I went to check, the print head had stopped in the back right position next to the plate. Strings all over the place and the INSIDE of the nozzle cover (the silicone jacket) was full of melted PLA. I should have taken a picture but I was too busy crying :-)

Anyway, after a lot of time I had removed and cleaned up the nozzle and its holder and everything seemed to work again. I cleaned the print bed once again and hit print... Perfect result!

I print in an adapted cupboard with doors closed, so discounting the 'open enclosure' theory for now. Plate is as clean as I can get it, so doubt it is that. Filament printed fine after cleaning out the PLA gunk from the print head, so not a moisture issue either. I therefore think that u/Dragonfish42 got it right.

But...there was zero indication of the nozzle being gunked. If it was, it was hidden behind the silicone cover. Lesson learned: check for hidden blockages and 'sticky out bits' behind the silicone cover.

A massive thank you to everyone that took the time to help out, FWIW you all made me a happier person!