This refurbished 850W Corsair Shift PSU is £84 vs £150 new from Scan UK
Complete with a one-year warranty.
Corsair's Shift series of power supplies are one of my go-to recommendations, thanks to their reliable power delivery and convenient side-mounted connectors, and now you can pick up a factory refurbished 850W 80+ Gold unit for just £84 from Scan, versus £145 for the very same PSU new at Amazon or £150 at Scan. That's a huge, nearly 50% saving, and well worth it - even for a factory refurb unit with a 12-month warranty.
The advantage of the Shift series is that it shifts the power connectors from the back of the PSU to the side, allowing you easier access when it comes to plugging and unplugging your modular cables. I find this is a nice feature to have for building and maintenance, especially if your case's PSU cavity is also home to SSDs, hard drives and fan controllers.
Of course there's an argument that you ought to go for a new power supply with a longer warranty period, and I think that's totally fair - it's about your appetite for risk. The safest option is to get a new power supply, and if you want this particular model than you'll have to pay a fair bit more - but the 10-year warranty ought to offer some comfort. If you don't need the side-mounted connectors - eg if your case has a minimal gap between the side of the PSU and the side case panel - then the cheapest 850W Corsair option is this RM850e for £99.