I managed to lock myself out of a file in svn - don't know how - but when I tried (re)-getting the lock (Tortoise was showing the "Get Lock" option for the file), it complained that already had the lock. I tried deleting the file and committing the directory change - same result. I tried CleanUp (including refreshing the overlay), but that failed too.
The solution was to go into the Tortoise repo-browser, find the file and use the break lock function.