Use the current deals on e2save (found by positvt). Go via quidco to get £7. Purchase the PAYG handset = £14.88 (So thats £7.88 cost to you so far). You can stop here but if you don't like the phone go via Quidco (for £4.50) to trade the phone in at Cashinmyfone. They will give you £5.05.
So all in all the maths work out like this £14.88 (phone and £10 top up) - £7 for Quidco - £4.50 (for using Cashinmyphone via quidcos) - £5.05 (for the handset) = final cost to you £-1.67 (profit) and a free £10 top up voucher.....Yaaaay