If you only need call minutes and text messages
without a new phone, call Vodafone 30 days before the end of your contract and
tell them to convert your line to pay-as-you-go at the end of your contract,
they will like to keep your business and will offer you a contract for about a
tenner a month with your present phone. (Depends on your bargaining power)

But if you need a new phone, take the route
advised by Rob.

My mate got a 12 month contract of 400 mins
and 100 texts for a tenner a month on three with his old phone.