I suspected for a while that my build plate was warped so did a full check with a dial gauge....
Worst along the X-axis 0.4mm
Worst along the Y-axis 0.21mm
You can download a copy of the results here (included is a blank form you may find usefull) -
https://www.yousendit.com/download/bWJwZGlwMHdZY1I4SjhUQw
