Update
I checked the output of the pendant using the teminal, thius eliminating my configs and Linuxcnc
What i found was the message
xhc-hb04: connection lost, cleaning up
Is generated randomly but often. As this is the same message you get when the usb is removed and my old pendant worked fine I am thinking its a fault on the usb device - or something different about this replacement and its interaction with the driver.
Im using linuxcnc v2.8 - I wonder if there have there been any HB04 driver updates that may help? - or do i just get on and return it to the supplier.