Many thanks Frenske,
Part 1:
I had mistakenly set up the expert mode to advanced thinking that was the same as expert. Once I'd changed that the Recording paths appeared in the Expert settings menu.
Here are the complete menu paths according to 7.2 terminology for anyone running into the same problem
Setup->System->Expert settings->User mode
Setup->System->Expert settings->Recording paths
Part 2
In the Recording paths menu the first two recording settings were set to <default movie location> (couldn't figure out where that is configured) but the Timeshift location was set to /media/
I don't know why that didn't work (perhaps a logical path to where the broken internal disk used to be) but when I created a new directory and used this instead then timeshifting started working
Thanks again!