Looked over BT Webmail site but nothing found. BT Yahoo helpful but no use.
If they are in your Trash folder you can restore them from there. If they are not there if you are a BTYahoo email account holder you could try as per this link
If you are a BTMail account holder, there is no way to recover them once they are deleted.
If you are not sure which system you are on, log onto your email account and look at the BT logo. If you see Yahoo you are on the BTYahoo system.