I think I've stumbled on a good deal...or I've missed some small print somewhere.
Dialaphone have O2 SIMs 4 months half price, 600mins, unlimited text normally £20p/m but on offer for £10 per month via redemption. Its a 1 month notice contract. With topcashback offering £30 for sim deal. This works out at £10 for the whole 4 months! ok bit of hassle with the redemption side of things.