The 6 year limitation within the Consumer Rights Act 2015 applies to 'finished goods.' A motherboard is a component part that I doubt would be included. Implicit in the sale of components is the condition they will be fitted by a competent person. Good luck arguing that a competent person would continue to use a component they believed to be faulty.
As someone on the other side of the fence as it were, in the business of operating an independent retail IT workshop. It is quite scary to read threads like this that highlight what a minority of customers think they are entitled to. Honestly, a £120 parts purchase, 28 months down the line, you believe it reasonable (in Law) for the retailer to remain liable? Thankfully it is not the case.
Generally speaking customers have 30 days to raise an objection after which they are deemed to have 'accepted' a transaction, warts and all. For instance should a customer RMA a part and it is returned no fault found, by not raising an objection within 30 days the customer infers agreement to there being no fault.
Generally speaking 'use' is a customer benefit deemed to have a financial value. Regardless of the presence of a fault by choosing to use the parts the customer chooses to take the benefit that reduces the value of their financial loss. The most the customer of a £120 motherboard can lose is the £120 purchase cost and £20 of that was VAT. After 28 months of use the potential loss is (probably) near zero.
Manufacturer warranties are a grey area as the customer does not have a contract with the manufacturer at point of sale. For a manufacturer warranty to be enforceable the customer needs to form a contract with the manufacturer, usually through a product registration process. In most cases manufacturer warranties include stringent conditions. For instance, fitting RAM with a part# the manufacturer has certified as working with a particular model of motherboard.A warranty period doesn't actually give any guarantee that the product won't develop a fault, it just says they'll cover the costs of repairing/replacing if it does do so.
Retailer warranties in the UK rarely provide entitlement beyond statutory rights and typically serve as a means to ease a sale. Legally speaking a retailer warranty is a promise, similar to the promises a salesman makes at point of sale. Insurance policies can come advertised as warranties. The contract is between the customer and an insurance company, with the retailer merely acting as the agent and not liable for the contents of the policy. Shady retailers may claim these insurances replace your statutory rights but they do not.
Should a customer believe goods are faulty the burden of proof lies with them; especially after 22 months. Customers are entitled to seek qualified independent testing but typically choose not to due to the cost. The retailer is not obliged to incur the cost by providing a testing service free of charge. Free testing by the retailer is a best efforts gesture of good will.So your only line of action I think is with the handling under your warranty claim. If the warranty conditions stipulate scan must be able to replicate it and they couldn't then that's probably that. If you have evidence that they didn't even try then perhaps that's another route, but that's unlikely.
You pays your money and choose your level of risk, is more accurate. When you pay a business to build your PC the business carries the majority of the financial risks. Carrying the risk is a large part of the service customers are paying for. Retail IT is challenging due to the complexity of the product, high capital values and wafer thin margins on top. There are certain goods and services my business could sell but does not as I can't (yet) afford the potential liability.Unfortunately you pay your money and take your choice.
I fully understand how individual customers can end up feeling aggrieved when they discover both parties to a purchase contract are taking a financial risk but it is a little disappointing to see unreasonable expectations being reinforced. Consumer protection laws in the UK are excellent and firmly on the side of the customer as it is.
Disclosure. I have used Scan in a private and professional capacity for at least 10 years. My current business places a couple orders a week with them. In my experience Scan are one of the easier IT suppliers to deal with in general and when things don't go according to plan. We have had the odd problem over the years but on the whole, they are pretty good in going beyond what they are obliged to. The balance between service and price is excellent, in my opinion.
I am not a fan of Amazon and the service we get is very mixed. Amazon are often accused of bullying market place sellers into un-viable business models, encouraging sellers to set up to sell as much as possible as quickly as possible, with every intention of going bust. Rinse and repeat.
To be clear. IANAL. Buy where you will.