Tried a Log Backup - unsurprisingly that failed.
however, a Full backup was successful. I'm a bit surprised as I assumed it would append the LOG file to the FULL backup, but perhaps there wasn't anything "active" in the damaged area of the file.
Perhaps I could just restore that FULL Backup back over the corrupted database (log file)?
I'll try restoring it to a TEMP database and seeing if it looks like it is OK.
Presumably if I rename the damaged database ("belt and braces" in case I need to access it) that would also carry with it all the Jobs etc. associated with that DB, so not really that good an idea to rename it ...
... perhaps I should stop the SQL service and do a raw copy of the MDF / LDF files as a "belt and braces" (after checking that my backup, restored to a TEMP database, seems to be OK)