After stopping all non essential running processes (itunes, malwarebytes, etc)... i tried again. it failed.
following tips online, I uninstalled malware bytes, the only non MS security app i have, tried again, it failed
working through the list of tips, I ran windows update, installed the updates, tried to upgrade to W10 again, it failed.
I installed windows update trouble shooter, which apparently fixed a few things, tried the upgrade again, it failed.
ran the trouble shooter again and managed to sort out one problem it highlighted but couldn't fix (deleting a 4 year old update)... rebooted, tried to upgrade to W10 again and after half an hour of it going through its checks and stuff, said "you need to remove MS Security Essentials". Uninstalled that and it seems to getting a lot further. Slightly annoying that MS's online advice was to remove all
non-MS virus and malware stuff, only to be told part way through the upgrade to remove their own security app too.
would have been quicker to do the clean install rather than the upgrade.
[edit] finally a successful upgrade! what a palava... but at least it seems to have kept everything, bookmarks, logins, software, etc.