I have been running /public/upgrade/complete_migration.php and things have improved since I installed php5-cli (which run_complete_migration.sh complained about after I tried running it manually).
I can see some of my data now but running complete_migration.php does not seem to be improving things any further.
So at this point it looks like I am stuck with a partially successful upgrade ... People (i.e. contacts are broken and have tags from other areas added to them).
Thanks for the reply, I'll snapshot my VM again and revert back to 1.75 unless anyone has any ideas on how to progress further