Can anyone help me please? I've followed this procedure
several times now and checked my password, and it just won't verify my calendar.
Are you on the BTYahoo email system or the BTMail email system.
If you are not sure you need to log onto your email account via a web browser and if you see Yahoo anywhere it means you are on the Yahoo system.
If you are on the BTMail system the link you posted is not correct it is only for the BTYahoo email system and even at that I would have doubts that it is still relevant.