I’m going to try moving the end stops first and see what that does. I’m recompiling the firmware anyway as I want to change the maximum bed size from the default of 200mm (X_BED_SIZE) to 590mm as it was something I should have done months ago. Amazing how long you can tolerate silly issues 
If that doesn’t work the with end stops I’ll investigate the SKEW_CORRECTION.
From memory I think the SKR V1.3 allows saving to EEPROM. I’ll find out very shortly once I work out how to transfer the new firmware via SD card. Normally I’d just attach a laptop but its in the garage, I can’t access the jumper on the SKR MB as I forgot to leave a slot in the case when I built the case . So now I have to export it from platform.io to an SD cad and can’t recall how to do that either.
(Figured it out, see here for neat way to do it Simple way to get platform.io to write to the SD card)
It’s just one thing after another today
Rob
[edited for clarity]