Are you looking for mail which is sent to your ntl address to be redirected to Yahoo? This would need to be done at the ntl mail server and hence could be impossible. Most ISPs will let you retain access to your mail either via Outlook or webmail for a period after the end of your contract, if this is the case then all you need to do is let everyone know to use your new address and then mop up the stragglers in the first month of your new contract.
EDIT: Another option (alongside the above) would be to set the Reply To address for your NTL account in OE to the Yahoo address, that way anyone who just hits reply sends it to Yahoo.