Honestly, you shouldn't bother with overclocking or watercooling if you're not confident with building PCs.
Breakdown of what I would recommend:
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CPU: NEW! Intel Core i5 4690K - For £1300 you should ditch the watercooling and the overclocking and simply get a fast stock processor with a bigger cache, such as the Core i7 4970K, which is this fast with no need for overclocking or watercooling.
CPU Overclocking: 4.3Ghz - Extreme Overclock - Unless you know what you're doing don't ever buy something overclocked this much, as you will not know how to deal with stability issues and the lifespan of the CPU will be much shorter.
CPU Cooler: NEW! Corsair H55 Water Cooler - Watercooling is an expensive system that just isn't required when there are quiet, effective air cooling solutions out there for 1/5th the price. (Look at the Cooler Master Hyper 212 Evo)
Operating System: Windows 8.1 (64-bit)
Motherboard: NEW! Gigabyte Z97X-Gaming 3
RAM: 8GB Corsair 2133mhz Vengeance (2x4GB) - For this kind of price range you should really be looking at 16Gb of RAMM. Games are already being released with a minimum requirement of 8Gb, won't be long before this goes up. Also, there's no real reason to overclock RAMM, you get basically no performance increase and lower stability.
Hard Drive: NEW! Kingston 120GB SSD - 120Gb is really not enough for gaming unless you only play 1 or 2 demanding games. Remember with SSDs you need to leave half of the memory free in order to get the best performance out of them.
Secondary Hard Drive: 1TB S-ATAIII 6.0Gb/s
Optical Drive: 22x DVD±RW DL S-ATA
Graphics card: NEW! Asus GeForce GTX 970 4GB Strix OC - 970 is probably the best card for your price range.
Sound card: Onboard 7.1 Audio
Internet: NEW! Gigabyte Wireless 802.11AC 867Mbps + Bluetooth PCI-E Card
Case: NEW! Corsair Carbide Spec-01 - Corsair make great cases.
PSU: NEW! 650W Corsair CS Modular - For gaming you want to be looking at the RM series as minimum, especially in any rig that involves overclocking.
Keyboard and mouse: Zalman M300 Mouse and K400G Keyboard - Input devices are always personal preference, but I would recommend trying a mechanical keyboard, you'll never go back afterwards.
Monitors: NEW! Iiyama 24" GE2488HS-B1 Gaming LED (HDMI, DVI, VGA) - BenQ are the best gaming monitors right now, but you do pay a premium for them. Just make sure you get something with 1ms or 2ms response time to avoid screen tearing and eye strain.
Remember, it all depends what you're playing. Arma requires less graphics power and more CPU, RAMM and Hard Drive speed/size. Whereas games like The Witcher 3, Elite: Dangerous etc, require more graphics power.
Hope this helps.
EDIT: Try 3XS System from Scan.co.uk as well. Very good team and great customer service with a 3 year warranty.