On offer at the moment at mobilephonesdirect.co.uk is the Motorola G4 @ £10 per month. The phone is free and comes with 500 mins unlimited text and 1gb of data I think this is a good deal. It works out about £4.60 per month after the phone is taken out. Great deal I think. And you don't have to pay up front for the phone.
£30 Quidco too! http://www.quidco.com/mobile-phones-direct/?