So for anyone reading this - please if you are using a Tevo Tarantula (or an extruder from one), check the bowden tube isn't pulling back into the extruder gear on retractions!
Yep, almost 100% sure this is the issue that started this whole mess! Then swapping in a stepper motor with different wiring at the plug caused me to blow the driver! Ahhh. Lesson learned (£170 lesson sadly - new TMC2660 chips and eventually a new board). On my setup it's impossible to see into the drive gear so I couldn't tell this was happening and when extruding the mangled bowden tube then pushed back to where it was meant to be but almost completely closed stopping it extruding properly! No idea how I got away with it working for a few years as it seems a flawed design!