Is it my PSU causing my issues?

That graphics card has a system requirement of 450 watts. So even when it was new, the power supply was under powered.

I would recommend switching it out for a 500 or 550 watt power supply. Then give overclocking a try.



Seasonic FOCUS series SSR-550FM 550W 80 + Gold Power Supply, Semi-Modular, ATX12V/EPS12V, Compact 140 mm Size, 7 yr warranty
https://www.newegg.com/Product/Product.aspx?Item=N82E16817151203
 
Solution
Here is the UK version. Also since the prices in the UK are high you may want to consider these as well

Seasonic FOCUS series SSR-550FM 550W 80 + Gold Power Supply, Semi-Modular, ATX12V/EPS12V, Compact 140 mm Size, 7 yr warranty
UK > https://www.newegg.com/global/uk/Product/Product.aspx?Item=N82E16817151203




Also since the prices in the UK are high you may want to consider these as well

EVGA 220-G2-0550-Y1 G2 80 Plus Gold Rated 12V ECO Mode Power Supply
https://www.newegg.com/global/uk/Product/Product.aspx?Item=9SIA24G3RH5192

Antec Earthwatts Gold Pro Series EA550G Pro 550W Semi-Modular, 80 PLUS GOLD, 120mm Silent Fan, PhaseWave Design, 7 Year Warranty
https://www.newegg.com/global/uk/Product/Product.aspx?Item=9SIAD6H6MW3706&ignorebbr=1