Found this offer whilst looking through their website. Granted its a 24 month contract, but the fact this phone goes for £120+ on eBay, doesn't have Vodafone's programming on the phone (checked today) and you can get 5 lots of cashback at £21, effectively making it £10.62 a month.
This deal is also availible for the Nokia 5800 but with no cashback.
*EDIT* £42 QuidCo aswell. 24 x £15 = £360 - £104 = £256 - £42 = £214 = £8.92 /month.