Depends, up to the Courts really. The current liability period is up to 6 years. However, up to 6 months after purchase it would be up to the retailer to prove that the goods were not inherently faulty, after 6 months the burden of proof switches to the consumer (to prove the goods were faulty and should have lasted for longer). If faulty the retailer must offer a repair, replacement or refund (depending on what is the most acceptable/economical).
If you can't manage to agree on a course of action with the retailer than you have to sue them in the civil courts, it will then be up to the court to decide what remedy, if any, is suitable.
These all come under the Sale of Goods Act 1979 (as amended).
However, this is all set to change within the next few years with the incoming Consumer Rights Directive, which will change the liability period to a guaranteed 2 year period (but no more than that). This Directive will be a maximum harmonisation Directive too, which doesn't give Westminster the scope to extend the liability period either.
