But surely you also have to see it from the supplier/manufacturers side. Putting the warranty issue aside for the moment, how do they know you are sending them the same card back that they sent you?
If this were a 10 year old card with a lifetime guarantee I'd agree with you.
The 980ti released approx 4 months ago.
There is no 980ti in existence that is not in warranty. I'm sure MSI can see it is an MSI card, and so, even if it were not the one purchased by the OP from OcUK, it is definitely one produced by them and still in warranty and they should repair/replace it.
In fact, as the card is under 6 months old, it would be deemed an inherent defect and down to the retailer to disprove as faulty rather than the other way around were it to go to court, so why bother trying to fight it? To upset the customer and ensure they never shop there again?
Were I the OP I'd just send a letter before action and if no positive response go to small claims.