Having put the 4 port serial card back into boron, the onboard port now works again, so I’ll probably continue with developing the software. The expansion card still doesn’t work though, so I’ve ordered a cheap 2-port card from eBay in the hope that a different card will work.
Before reinstalling the card I upgraded Ubuntu to see if that would help (it didn’t) which brought its own scary moment of the 1TB RAID volume being dead. That too is solved now – the drive letter assignments had changed.