A journal will work fine although in most cases you can just go back and edit the purchase invoice to change the code.
If you have a number of incorrect expense postings to relocate you can use our new bulk re-coding tool:
http://community.quickfile.co.uk/t/bulk-change-incorrect-postings/4984
Tagging is used in a number of different contexts but usually refers to the process of going through your bank statement and explaining your transactions. E.g. you may have an untagged “money in” entry on your bank, if this were money from a client you can quickly tag it and QF will create the backing invoice.
See here for more information on bank tagging:
http://help.quickfile.co.uk/main/1/tagging_your_bank_transactions.htm