For completeness / in case others come across this, turned out the XML export file was being corrupted during transfer between the two systems. It was a CR/LF corruption - in our specific case CR/LF was getting converted to CR/CR/LF !!

Highlighted that DTL's specifically appear sensitive to CR/LF changes - we've subsequently come across the same #6301 error loading DTL's get CR/LF modified in another way (we're still tracking that one down, though suspicion currently is with git...)