Looking to build a gamingPC £400

With that budget, I don't see you getting a gaming PC.

Now that I can work with. :lol:

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: AMD FX-8320 3.5GHz 8-Core Processor (£107.93 @ Amazon UK)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler (£23.90 @ Ebuyer)
Motherboard: Gigabyte GA-970A-UD3P ATX AM3+ Motherboard (£68.34 @ Aria PC)
Memory: Kingston Black Series 8GB (2 x 4GB) DDR3-1600 Memory (£60.60 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£42.00 @ Aria PC)
Video Card: XFX Radeon R9 290 4GB Video Card (£299.98 @ Amazon UK)
Case: Cooler Master K280 ATX Mid Tower Case (£32.38 @ Amazon UK)
Power Supply: XFX 650W 80+ Bronze Certified ATX Power Supply (£65.99 @ Aria PC)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer (£11.98 @ Ebuyer)
Operating System: Microsoft Windows 8.1 - OEM (64-bit) (£85.04 @ Amazon UK)
Total: £798.14
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-01-10 18:06 GMT+0000)

A cheaper option will be:

PCPartPicker part list: http://pcpartpicker.com/p/2wrFC
Price breakdown by merchant: http://pcpartpicker.com/p/2wrFC/by_merchant/
Benchmarks: http://pcpartpicker.com/p/2wrFC/benchmarks/

CPU: Intel Core i3-4340 3.6GHz Dual-Core Processor ($159.98 @ SuperBiiz)
Motherboard: MSI H81M-P33 Micro ATX LGA1150 Motherboard ($49.99 @ Mwave)
Memory: Crucial Ballistix Sport 4GB (1 x 4GB) DDR3-1600 Memory ($39.99 @ Amazon)
Storage: Seagate Barracuda 500GB 3.5" 7200RPM Internal Hard Drive ($52.99 @ NCIX US)
Video Card: MSI GeForce GTX 760 2GB Video Card ($249.99 @ Microcenter)
Case: Zalman ZM-T1 MicroATX Mini Tower Case ($27.99 @ Mwave)
Power Supply: Corsair Builder 430W 80+ Bronze Certified ATX Power Supply ($19.99 @ Microcenter)
Total: $600.92
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-01-10 14:47 EST-0500)

You forgot the OS. Also, I think the i3 is not going to hold up very well in next gen titles. Crysis 3 already is showing that. THG recommends an FX 6300 or an i5 quad at minimum. I do not see the multicore support trend going away given what the PS4 and Xbox One have for hardware. The power supply doesn't leave really any headroom for upgrades. Single channel 4gb of memory would hurt performance too. 500gb would fill up fast with the size of some games. My 750gb storage drive on my main rig is nearly full just from games. WoW alone I believe is 20+ gb. The GPU is a pretty good upper midrange card.

I am not so sure about future titles, but as for now it performs better then than FX. True about the PSU, i dont know if he is planning on upgrade. You also could but more storage in the future, but for budget reasons I didnt choose a 1tb HDD

Surely quoting everything in $ is a bit false economy though?

This, I can guarantee if you entered the same parts into uk.pcpartpicker it would cost much more.

These kids are spoiled and foolish if they think that you can't build a perfectly functional Gaming Tower for under $500. First off I'd like to say that your initial build will not have all the bells and whistles, so to speak. You'll have to start off small and leave your self some room for future upgrades. I saw you mention that your wiggle room is up to $800, but you said $400 at first so I'll aim for $600. Something important when you're on a tight budget is to shop around for your parts and find the best prices. I recently built my self a desktop that i spent $160 on and I can run all of the latest games. Some of them on low settings, but i don't give a fuck as long as it runs them smoothly. I also had a case, a video card, and hard drives already so that cut my cost quite a bit.
I'd recommend going with AMD since you're on a tight budget.
Processor: FX-6350 Average price: $140 on Tiger direct
Memory: 8 GB 1600 C9 2x4GB Average price: $80
Case: Z20-4010 Zalman Z9 Plus ATX Mid Tower Case Price: $66.17 on Tiger Direct. Chosen for the fan controller, temp readout, and 4x 120mm fans.
CPU Cooler: Any $30 cooler master cpu cooler will do.
Motherboard: A455-9701 ::ASUS M5A97 LE R2.0 Price: $85 on Tiger direct
Power Supply: Ultra LSP550 550-Watt Price: $35 on Tiger direct
Hard drive: WD Blue 1TB Drive Price: $70 on Tiger direct
GPU: GIGABYTE GV-R775OC-1GI Radeon HD 7750 Price: $80 on Tiger direct
Total: $600 Shipped

Oh and I'd Pirate my OS, but that's just me. You can buy Windows 8 for about $85 if you want. Which after you shop around and cut your costs will still keep you under $600.

After a few months if you have some more cash you can feel free to upgrade by adding a bit more RAM, Upgrading your video card or grabbing another one to crossfire it, adding another hard drive, and eventually grabbing a new processor. Especially once AMD comes out with their next gen of AM3+ processors.

Guaranteed these are not at all the best prices that you can get for these items. Spend a week looking at craigslist, ebay, and google to find the best prices for these items or comparable items. There is nothing wrong with some used RAM in a new system(especially when most RAM comes with lifetime manufacturer replacement). I'm also confident that you can find better prices for your Motherboard, CPU, Video Card, and Hard drive. I'd stick with a brand new Power supply from a trusted source though. I personally cut all my costs on my system by 41% just by shopping around. Good luck. Hope this helped.