The law gives you up to 6 years to get a problem remedied by a retailer (either by a repair or by a replacement) as long as the cause is an 'inherent fault'. That refers to something which was wrong with the phone at the time of purchase, such as the use of poor quality components or poor workmanship. For the first 6 months after purchase there's an automatic assumption (unless there's clear evidence to the contrary) that any problem must be due to an inherent fault. Thereafter it's up to the purchaser to show that, based upon the balance of probabilities, an inherent fault is the most likely cause of the problem.
Go back and demand that Asda complies with your statutory rights.
Chris
PS: I bought a mobile phone from Asda which had the seal broken on the box. (The one which says 'do not accept this purchase if seal is broken'). I decided that, because I was buying from a reputable retailer, there would be nothing to worry about. Shortly after that I started getting lots of phone calls from debt collectors, chasing a 'Debbie England' whom I'd never heard of. Then I got a call from Miss England's boyfriend, threatening to beat me up for nicking her phone! And you think that YOU'VE got problems!!!
;-)