The post by
@Benass is about the most relevant to the problem, though it confuses a few things.
Your warranty with the computer store was only 12 months. This period has passed, ergo there is no warranty. However, you do have some statutory rights which you could pursue (these
do not constitute a warranty or guarantee). Under the Consumer Rights Act (2015), goods should last a "reasonable" period of time. It would be hard to argue that a £2,500 PC should only last 14 months. However, I expect you will have a fight on your hands. Just because you
may have some rights under the law, it doesn't mean that those rights are easy to exercise.
If you try to enforce your rights under the CRA, the retailer is likely to ask you to prove that you didn't cause the fault. They have every right to do this. So you may as well get a second opinion now, and ask the other shop for a written statement detailing the fault. Even if you have a written statement from this third party, the original retailer may not back down, and you'll have to look at Alternative Dispute Resolution (ADR) or court action to have your rights enforced.
How did you pay for the PC? Credit card? Debit card? Cash? Finance? Section 75 of the Consumer Credit Act could help here, as it makes credit providers jointly liable for CRA claims. Chances are, you'll find a finance company or credit card provider easier to deal with than the retailer.
Forget about OCUK and Zotac. They're pretty irrelevant to the situation.