Andy,
Just about to have tea, so a short answer :-)
As I remember, when you select "Delete Old Data" from the MyLog > Tools menu, you get the option to delete ALL you MyLog data, or all data before a date you can select.
Top tip here - shut down RB, make a copy of your MyLog.db3 file in the /Data directory in case it all goes horribly wrong, then restart RB and give it a go!
Your main source database (NavData.db3) is unaffected by this, only the MyLog record of what you have picked up "Live".
Anything more clever that this, and tarbat is your man!