Phones4u have a range of phones that have just appeared on clearance on o2. They have a number on £4.99 per month for 12 months on 12 month contracts (see http://www.phones4u.co.uk/special-offers/Various2.asp)
including Samsung E900, LG Chocolate KG800 (White, black, red/wine) - these work out at just short of £60 for the year (-£32 Quidco = £28 for the year) for o2 400 (400 mins x-net/100 texts). Claim by cashback (1 claim in month 6).
The best offer appears to be for the Motorola RAZR V3 Black Clearance. It is available on o2 400 tariff (£35 per month, 400 mins x-network, 100 texts), 12 month contract at 99p per month (Discounted line rental redeemed via £408.12 cashback in month 6).
Cashback claiming details on their website at http://www.phones4u.co.uk/help/faq-cash-back-and-redemptions.asp
Send voucher, bills m1 and m6 in month 6 - only 1 claim.