If you are looking for a new "better" contract then just call the usual Customer Services number, when they ask you to say why you are calling just say "I'm thinking of leaving". That will get you transferred to the "Retentions Team", also known as the "Loyalty Department". That will allow you to negotiate a new deal. Decide what you would like in advance and negotiate for what you want.
Not saying you will get exactly what you want, but if you don't ask you definitely won't get .