well it looks like the problem is solved, thanks to gambo :-)

Apparently over time the pid settings somehow start to effect the hysteresis settings and the printer cuts out.
Hysteresis is right - basically she's having a panic attack.

feflash the dfirmware and reset the pid settings - and here's a golden tip for you: when you get a good set of pid settings, save them in a text file for easy access and really quick flashing to the firmware.

So i dropped the original marlin firmware on, ran a dry print (no filament) and the mould that had died 3 times ran all the way through.

So I've put the medellist firmware back today and reflashed my good pid settings and am currently coming up to the critical phase of another dry print.

If that goes through without any issues I'll do a 'wet' print over night :-)

And i changed the nozzle to a hardened steel one I've had gfor a few years and never used. still 0.4.

And i discovered that brass 'standard' printer nozzles are nothing of the sort.

All the cheapo ones I've acquired with bits in the last few years are a smaller socket size than anything in any of my printyers. WTF ????
I can't recall ever seeing a required socket size on nozzle listings.

Bloody weird.