Well done Mike you are right on the money! I looked very carefully at the part number on the TVS diode on the old boards vs new and it appears different. It should be a SMBJ170A TVS diode, though the old boards show a PN of B711 PR, and the new faulty one shows P836 PR - see below:
I've Googled both of these and can't find a datasheet for either of them so I can't be sure what the difference is. However I tried desoldering it from one of the new boards (kind of ripped off the track along with it but anyway it's gone. And the problem seems to have vanished so it looks like this is the problem.
I guess I'll need to go back to the manufacturer and see why they deviated from the stated BOM... If anyone is able to find out what the PN I have in the board actually is I would be interested to hear 