Only thing that is gonna stop a determined thief, is an Almax and an Abus, and a lampost, chain and lock wise, but thats no way under £500. Anything under £50 will only be a visual deterrent, which is what deters most anyway. The alarm disc locks are pointless, and as a lot of bikes are stolen from parking bays by two men and a van, any disc lock is useless, they just pick it up, chuck in van and go. I have plenty of cheapo chains and locks, so if you just want a visual deterrent, pm me, it's Xmas, you can have one gratis.
