OK, this is stupid, but I cannot for the life of me find the revision history view in TortoiseHg Workbench. It was there and then it was gone.
It lists each revision and then it is possible to click on a particular revision, causing a list of all the files (of that revision) to appear in a left column window.
I feel like I have tried every button and menu.
Default WorkBench view
If you have some panes hidden (collapsed?) you have to find border and drag&drop it in the right direction (sorry, I can't show cursor on screenshots).