Saw an ad in the paper this morning (the Metro).
O2 are selling:
Sony Ericsson C902 for £97.86 (offline and online)
Sony Ericsson W595 for £78.26 (appears instore only)
The C902 is a bargain at this price and there is also a possibility that they will let you trade in an old phone for extra money off (instore) and give an upgrade discount to existing customers (5% of top ups over last 12 months I think).
If ordered online then Quidco will apply.