It's too early to tell Dave.
There are a few possibilities here:
GPU knackered: Not under warranty, replace with the 980 I'm sending you, gain an extra 7% FPS.
Motherboard knackered: I have the receipts and stuff, send it back, win the internet.
I've got that 980 in my bag here, post office opens at 9. I haven't had time to clean it unfortunately so it's a bit dusty but it'll be with you tomorrow.