Watch the mobile bills. I thought my tarrif was fine, and then realised that every time I went over my minutes by an hour or so, it cost me around £25. I've gone with one of those cashback deals given that none of the networks can get even vaguely close to them...
IMO one of the biggest areas you can save cash on is food - it's possible to eat well and get a few beers in for £25 a week if you plan your meals so that you only buy what you need and cook from scratch, and if possible get your meat from a butcher, as they're often a lot cheaper than the supermarket, and have cheap tasty cuts that you can't find in the supermarket. OTOH the ready meal we had from M&S last night worked out at something daft like £5 a head.