Occasionally I commit some code to the repository, add a comment/summary, then read the summary back and realise I\'ve made a mistake or should have included a bit more info
TortoiseHG as long as you had not pushed:
Mark latest revision (which is to be ammended),
click the green tick on top as if you would like to commit.
Open the menu triangle right from the "commit" button on the right hand side and choose "ammend current version" instead of "commit changes".
Here you are.