Ok… so I read through all the firmware updates and purposely have been hanging fire from upgrading as i had a pretty good know baseline i could work with. Ive also read pretty much of all the bed levelling threads too some great info in there @Tone
So upgraded to 1.10.1 from version 1.8. Rebooted… Ran calibration which worked… though interesingly the nozzle is much closer to the bed when doing the final manual calibration than with 1.8 (like about 8mm nearer… relevance will come in a minute)
Started a print of a Gcode file that printed fine yeterday with 1.8 and the nozzle was very well rammed against the bed… so quickly tried to do an offset to lift it a bit and it behaved weird and ended up diving further down and realllllly gouging the bed even though i was going up…
Anyway upshot is Ive had to put a G0 Z7.7 into my Gcode that worked yesterday, followed by G92Z0 to basically shift the Zaxis origin up nearly 8mm so that it actually prints correctly.
What is going on?! - just in middle of a print now that seems to be working ok so will do a M420 V later to get the bed mesh points