How long is the warranty on your power supply? If it's 10 years, use the card. If it's 7 years or less, replace the power supply. 7 years old means possibly an old group-regulated design, which isn't ideal for a 529 quid 3000 series card. No warranty means you're out of luck if it messes up any other hardware. Sure, you have more chance it'll work fine for quite a while before anything bad may happen. But I know if it was me, I'd want that peace of mind and if I have 529 quid for a new card, I should also have 100 more quid for a decent power supply.
You have MM access, I've seen a few decent power supplies go for reasonable prices on there recently. I'd check that out.
CPU at 1440k no real problem. Okay, you may not see the FPS counter as high as with a more recent chip. But you
will see a noticeable uplift and as
@Aretak says, overclock it to at least 4.5Ghz and you'll probably find you're pretty happy running with it for the time being. At 1440p, your choke point becomes your GPU.
Keep it, use it, maybe even mine back some of the money for it and put that towards a system upgrade.