this should do the trick, reputable brand and 7 year warranty.

My basket at Overclockers UK:

• 1 x Super Flower Leadex III 550W 80 PLUS Gold Modular Power Supply - Black= £59.99

Total: £70.49 (includes shipping: £10.50)
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As above, most people recommend superflower

 

Vouch for the superflower 550W

 

I use a cooler master masterwatt lite 500W with my RX580 but have been told it's not recommended, even though it works fine lol.

After lots of research I came to the conclusion if I had to upgrade it would be the Corsair RM750X