Ok we are starting to understand your setup. Your talking about Windows live mail the desktop program not windows live mail the email provider.
Your email provider is btinternet.com who uses Yahoo mail. The windows live mail program is setup to access your BTinternet.com acccount.
We need to establish where your contacts are stored. If they are only in the window live mail program locally or are stored in your btinternet.com account online . Can you log onto your btinternet.com email account (not via your wlm program) and then look at the contacts section to confirm whether they are stored online or not.