There are two separate things here. There's the guarantee, issued by the manufacturer. And there's the Sale of Goods act, which say things sold must be fit for purpose.

Guarantees only apply to the original purchaser, so the paperwork demonstrates that is you (and you didn't buy it second hand). They're operated by the manufacturer, so it's always worth going back to them.I went back to Le Creuset, without paperwork, who pointed out that the lifetime guarantee had only come in about 30 years after I'd been given the pot in question, but they offered me a very large discount on a replacement (and are now getting their reward because I'm telling the world about it).

Sale of Goods Act - fitness for purpose applies irrespective of any guarantee and out of guarantee period, but it's a duty on the supplier. So you go back to your supplier, and you will need proof of purchase from them, and point out that a stainless steel fork that packs up after little more than a year of normal digging is not fit for purpose, and they have to give you a remedy, which I think (but may be wrong here) is a replacement or a serviceable repair. They'll argue that it's the manufacturer who you need to speak to - it's not, it's a legal requirement on the supplier to provide goods fit for purpose. Then they'll say it's out of guarantee - which is irrelevant. The goods have to last a normal lifetime, and one expects garden tools to last a good few years.