Qidi responded very quickly to my enquiry and sent an update kit for the carriage. I dont know why the carriage has an effect on the z axis starting point but I fitted the new kit and it has worked faultlessly since and I have done a lot of printing. Not only thay they followed up the bits with an email to check all was ok. The only issue I had was I could not get some of the screws out (very small heads) so had to change a bit more stuff BUT it was in the kit. Lots of extra bits in the kit so was not worried.