Smart Telecom Disconnect charge

Got an invoice from Smart Telecom yesterday with a disconnect charge for 34.99 incl. VAT. I moved from Smart to BT in November and received a letter from Smart confirming changeover and no mention of a disconnect charge. Has anyone else received a disconnect charge and how did they deal with it?
Thanks

What do the terms & conditions of your service agreement with Smart say abour disconnection charges? Did you terminate a contract before the minimum period (e.g. often 6 or 12 months)?