A guarantee isn't part of your statutory rights; it's a 'gift' from the manufacturer (or, occasionally, from a retailer). So it's entirely up to discretion of the manufacturer (or retailer, as appropriate) as to how the guarantee operates.
However I've never encountered any guarantee which commences anew at the time of a repair. It's normal practice for the guarantee to continue running from the date of purchase but a few guarantees simply cease to exist when a repair is made. (i.e. the guarantee effectively states "We'll repair it ONCE if it goes wrong during the relevant period but thereafter you're on your own mate").
Your statutory rights with the retailer (not with the manufacturer) are entirely separate and still start from the date of purchase. i.e. if the product goes wrong within 6 months from the date of purchase there's an automatic assumption (unless there's clear evidence to the contrary) that the problem must be due to an 'inherent fault' and the retailer must provide a repair or replacement. (An 'inherent fault' is something that was actually wrong with the product at the time of purchase, such as the use or poor-quality components or poor workmanship). From then on, up until 6 years after the date of purchase, the customer must show that an inherent fault is the most likely cause of any problem that occurs in order to be entitled to a repair or replacement.