Sorry, I've been away. iRex think it may be that the digitiser has always been faulty, just masked by the previous software. Anyway, it goes back, so with any luck:
a) The problem is as they describe and fixed.
b) The problem is not as they think but is a software fault & they now have a device that manifests the problem. This would help everyone as they can then try to solve it.
c) I might get a device with less of a calibration issue
Either way, until the box comes and it gets returned, fixed & back. I ensure ensure that if I am writing, the snooze mode is set to alongish timeframe & I save the document frequently so if it it does lockup I can carry on with little pain.