In effect, that's what I ended up doing.
When the initial install failed, it rolled back. Except the roll back did not restore the previous version of TC it had overwritten. So it left me with nothing. After the reboot, no trace of a TC install - except my script extensions folder. So I copied that out elsewhere, deleted the three or four leftover folders, and it worked.
No idea about the junit issues it's flagging up now. I don't use junit so as long as the rest of TC keeps working, I can live with it complaining about those.
Never had any issues with previous updates. They've always gone in cleanly ....