OK, I tried following (OS X Yosemite) the instructions here
And I went into Terminal and typed in the following
diskutil resetUserPermissions / id -u
But now it's been showing a line of little rotating slash marks for a good couple of hours. I wondered if it's just one of those processes that takes a ton of time like when you do some kinds of file repair or whatever.
I also wondered what would happen if I interrupt this and try restarting. I have a TM backup so worst case I can restore from there and try again.
The Mac BTW is a 2010 Macbook Air. So it's about at the limit of what can run Yosemite at all.