Let's separate the manufacturer and the retailer here to start with:
The MANUFACTURER isn't obliged to offer any guarantee at all. If the manufacturer chooses to do so, that guarantee can be as generous, or as limited, as the manufacturer wishes. (e.g. it's perfectly lawful to offer a guarantee that states that you can only get a repair or replacement if you return the product, in person, to the factory in China at 3am on Christmas Day while stark naked, standing on your head and singing the Albanian national anthem backwards).
However a RETAILER is obliged to repair or replace a faulty product if the problem develops within 6 YEARS of purchase AND that problem has arisen because of an 'inherent fault'. (i.e. something which was already wrong with the product at the time of purchase, such as the use of substandard components or poor manufacturing standards).
Within the first 6 MONTHS after purchase there's an AUTOMATIC assumption in law that the reason that a fault has developed must be due to an 'inherent fault' (unless, of course, there's clear evidence to the contrary, such as the tyre tracks of the car that ran over the product!). So, if your phone had gone wrong 5 months after purchase you could simply have said "It's not working. Fix it or replace it". Tesco couldn't have successfully argued against your rights.
For the remainder of the 6 year period the purchaser must SHOW that the reason a problem has developed is due to an inherent fault. (Some sources use the word 'prove', rather than 'show' but that's incorrect; civil law is based upon 'the balance of probabilities', not upon 'proof beyond reasonable doubt').
Theoretically you would need to get an independent report, from a qualified expert, to show that your phone has failed 'due to an inherent fault' but that's clearly not possible (because it would almost certainly cost more than a new phone).
However simply saying to Tesco "Can you actually suggest any alternative reason why my phone has stopped working?" might be sufficient; if they can't, your suggestion that it's an inherent fault at the root of the problem might be regarded as the 'most likely' cause in law.
So really hassling Tesco hard, citing your '6 year' rights under EU law (and challenging them to suggest why your phone could have gone wrong other than in a way that was due to an 'inherent fault') might get you a replacement. However you'll need to be really persistent!