For those that remember, last year people could take out a Vodafone contract SIM only and get a free handset like the HTC Magic 1400 deg - http://www.hotukdeals.com/item/453089/free-phones-including-htc-magic-nok
Well I went online to upgrade and if you choose Phone Only, you can get great phones for free on a 12 month deal.
For example, I'm on the £20 a month but just got the Blackberry Bold 9700 for free. The phone is worth more than the £240 I'll pay over the next 12 months so works out completely free.
EDIT: People have confirmed this works on £10 a month. So sell the phone and Vodafone are paying you £130+ over the course of the year.