^^^ The Sale of Goods Act has, along with the Unfair Terms in Consumer Contracts Regulations and the Supply of Goods and Services Act, been replaced by the Consumer Rights Act 2015.
PC World are responsible for repairing or replacing your computer if the problem has arisen due to an 'inherent fault'. (i.e. something which was actually wrong with it at the time of purchase, such as the use of poor quality components or poor workmanship in its construction). If the problem had occurred within 6 months from purchase there would have been an automatic legal assumption that the problem was an 'inherent' one (unless PC World could have shown otherwise). Now that more than 6 months has passed though, it's up to you to sow that the fault was 'inherent'.
Theoretically you might need an independent report to provide such proof but simply challenging the retailer to offer an alternative view might suffice. (i.e. if you say to PC World, "The hard drive must have been of poor quality because there's no other reason why it should fail so quickly", they might be hard-pressed to challenge that view).
http://www.which.co.uk/consumer-rights/regulation/consumer-rights-act
Take it back to PC World but be prepared to hassle them a bit!