This offer is back again for the phone @ £34.99. but the £20 credit limit is still there. Simply use the wincanton discount (details below) and you can get the K550i for just £38.98 delivered with £20 airtime. £3.70 quidco will nearly cancel the delivery also.
On the link page, put wincanton as the company name, and OPOSO as the password. Then select the £20 PAYG discount, navigate to the phone, and add it to your basket