So, all was going well and I decided to do some firmware tweaks (Richard has posted a summary of these http://richardcaunt.co.uk/bigbox3d-firmware-tweaks/.) After making some changes (including doing some bed mechanical changes) I wanted to rerun the bed leveling but noticed on the third point (I have a dual) that the y axis ran hard up against the end of travel. On investigation I discovered that after the autohome the nozzle moved to 15,15 but only reported itself as being at 5,5. Anyway to cut a long storey short, it would appear that I accidentally selected the Zero home offset menu item and shifted zero to 10,10. Performing a "Restore fail safe" didn't fix the issue. Connecting to a PC and issuing a M501 showed M206 X -10.00 Y-10.00 Z-8.00. I therefore issued a M206 X0 Y0 and stored the memory and all looks normal again. Hopefully this will help others in the future. I think I will look at disabling the home offset menu so I don't accidently do this again.