My first build. One question needs answering.

According to my numbers, you can fit a system which is comparable to the one in the OP for about 100 quid less:

PCPartPicker part list / Price breakdown by merchant

Type Item Price
CPU Intel Core i5-4430 3.0GHz Quad-Core Processor £141.72 @ CCL Computers
Motherboard Asus Z87-Plus ATX LGA1150 Motherboard £71.84 @ Scan.co.uk
Memory Crucial 4GB (1 x 4GB) DDR3-1600 Memory £22.84 @ CCL Computers
Memory Crucial 4GB (1 x 4GB) DDR3-1600 Memory £22.84 @ CCL Computers
Storage Crucial MX100 128GB 2.5" Solid State Drive £52.76 @ Ebuyer
Storage Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive £38.34 @ Aria PC
Video Card Zotac GeForce GTX 970 4GB Video Card £257.94 @ Amazon UK
Case Zalman Z3 Plus ATX Mid Tower Case £26.99 @ Amazon UK
Power Supply Antec TruePower Classic 650W 80+ Gold Certified ATX Power Supply £59.98 @ Amazon UK
Optical Drive Samsung SH-224DB/BEBE DVD/CD Writer £10.19 @ Aria PC
Wireless Network Adapter TP-Link TL-WDN4800 802.11a/b/g/n PCI-Express x1 Wi-Fi Adapter £25.67 @ Aria PC
Prices include shipping, taxes, rebates, and discounts
Total £731.11
Generated by PCPartPicker 2015-03-07 00:38 GMT+0000

Indeed, it even has SLI support, due to the larger PSU capacity. Of course, more could be invested, but since you've got enough for a 970 but not enough for an SLI, I'm not entirely sure of where to direct it. What are you current peripherals? You might well be able to afford a significant quality of life improvement in that area with some of the (potential) surplus.

/r/CabaloftheBuildsmiths Thread