I think I've cracked it.
I suspect that the entire problem has been caused by a faulty thermistor in the hot end. I tried raising the extrusion temperature and, when I got to 225 deg the print stuck down. And this is PLA we're talking about, previous extrusion temperature was 195 deg. So, replacement hot end now on order from Amazon. Of course I will shortly have a whole pile of handy spare nozzles and, whoopee, a replacement magnetic mat. Lucky, lucky me!
BTW. That sorta answers my simple question about the balling of the filament on the nozzle. If the temperature is too low but hot enough to just soften the extruded filament then it will not stick to the bed but deflect upwards and start sticking to the nozzle.
Thanks for your time folks, back to the printer now as I have six Christmas presents to create for the family; time is flying as one of them lives in New Zealand and I live in Portugal!