What size radiator are you looking at? 120MM, 240MM, 280MM, 360MM?
As far as air coolers are concerned, I personally like the Noctua NH-D14 the best at the moment (Of the one's I've tried). Don't know where you are at, but I'm guessing somewhere in the UK, as I think that's the Pound symbol you used above. Here used/pre-owned NH-D14's can often be found for $25-55 USD. The NH-U14S and NH-U12S look like nice coolers as well.
Someone will likely mention the Cooler Master Hyper 212X or Hyper 212 Evo as well, while that cooler is not bad, it's personally not one of my favorites (due to lower performance and somewhat low mounting pressure in my testing of them). It's sort of the defacto entry-level cheap heatsink.
I've personally owned a lot of AIO coolers, and had pretty good luck with most of them.
Corsair H60 (The old V1 H60, not the newer one.) (Had to be RMA'd, pump started making a buzzing noise but still worked fine. (Air bubbles caught in the loop as far as I could tell.) Used the replacement for somewhere around two years if I remember correctly.), Corsair H80 (Still being used in backup rig, have had it for probably four years now and never had any problems.), Corsair H80i (Pump failed, had to be RMA'd. Replacement works fine though.), Corsair H100 (RMA'd due to malfunctioning fan controller.), Corsair H100i (Used it for four years, still works.), Corsair H105 (Just got a few months ago, still works well. Wish it had Link functionality to read pump speed and/or fan speeds though.)
Despite the issues with the one's I've had to RMA I still like the products and the brand overall. Probably because I've had no issues with having RMA requests responded to and/or fulfilled when I've had an issue with one of their products.
Cooler Master Nepton 280L (Don't recommend it. Has weird non-standard fan spacing, so difficult to find cases that can fit it if planning to use 120mm fans (it can fit 120mm or 140mm fans, comes stock with 140mm fans though.) This cooler is/was outperformed by my H100i on the same CPU, so it should outperform a 240mm cooler, but for some reason doesn't. Also, tubing is VERY stiff, which makes it difficult to mount in basically any system/case combo. One of the stock fans has been out of balance and making a rattling sound since the day I started using it. Had this unit for probably close to four years now, but have only had it in use for probably two of those years cooling multiple systems that I put in my HAF X case for benchmarking and overclocking. Some sort of quality control issue I would guess.)