Bingo, that worked, thanks! I had to borrow a Windose laptop - only use Linux here.
Why it works, I'm currently flummoxed. I briefly convinced myself it would be to do with the horrid eof padding that xmodem adds, but I can't make any of teraterm/minicom/screen/sx behave differently. Something shonky with timing? I think this bears some more experimentation (and yes, semi-bricking it multiple times) with some serial / usb analysers. In My Copious Spare Time.
James G7NWK