In reply to j0ntyg:
This appears to be an 'improvement' in the BT Yahoo services and I am getting this too. Unfortunately BT have been so busy getting into television and football deals they have completely messed up moving their email accounts from yahoo to their own servers for many users. Emails saying the migration was imminent came out last spring, but this has been severely delayed as something didn't quite work. A migrated account (another member of the family has this) doesn't show adverts except for BT ones!. There are some accounts not working awaiting migration, but talking to customer services is like nailing your own hands to the table.
I would use an IMAP client if possible to avoid tracking, annoying adverts and keep copies of your emails.