GK420d prints everything as text

Attempting to send epl appears to 'break' the printer. I have two printers that were both cheerfully printing ZPL using a windows generic text driver, that now will only print whatever is sent to it as text - this includes reset commands sent from the set up utility and an attempt to update the firmware resulted in the machine printing out the contents of the binary. A factory reset from the line feed button also has no effect. Does anybody have any ideas as I now have two machines that are effectively 'bricked'. A snippet of the epl that I sent to the machine is :

 

N

Q822,24

R40,0

S4

D15

ZB

 

 

A760,120,1,1,1,1,N,"DPD"

A735,080,1,1,1,1,N,"www.dpd.co.uk"

A706,033,1,1,1,1,N,"Sender"