Charged for cancelled contract

PetiteParis

:mad::mad:I've just found out that I've been charged by Tesco £20 per month for an old phone not been used since July 2013 - it's not that I don't check my bank statements but there was confusion over my husbands work phone. They have offered me £50 but that's not much - I still have a sim only contract with them and another handset contract with them. anyone know if I will stand any chance with getting the money back. I have a copy of my cancellation letter to them but nothing confirming it back to me? Thanks