Is it a 4790k or a locked 4790? If it's the k CPU, I'd go for it if the price is right. Not sure what it would be worth in quid but in USD that's easily worth a good $300-$350
That answers the question. If the price is right, go for it. The only real difference between Haswell and Skylake/Kaby Lake is motherboard features, slightly better minimum framerates for the latter and a bit more OC ceiling for Kaby CPUs. IPC and overall gaming performance are nearly identical. You might notice a difference in workstation tasks though due to DDR3 vs DDR4 RAM.
All interesting stuff, Moved away from Intel years from a dual core 3.0 to a Gigabyte 990FX with FX 8350 which still does everything I require. Also has 16gb 2400 ram and 1060 6gb.
Not sure what the market price is in UK, but I wouldn't even pay that in USD. I'd haggle for mid 200s max. Keep in mind that is 4 generations old now with no warranty. And it's used.
212 EVO is a great air cooler, if you ran it stock and had 2 good fans in push/pull it should run pretty cool. I'm clocked to 4.6GHz gaming in 1440p 144hz with H100i and hit around 55c, stock I hit around 50c