As an official Dell Vostro 3900 i5 deal poster I fully endorse this deal and give it my seal of approval

Nope, as said in previous deals, average prices to build this are:
£140 cpu - i5 4460 incl heatsink/fan
£35 motherboard, your choice of budget mobo
£14 4GB memory, Crucial
£32 500GB hard drive
£25 cheap n' cheerful case
£18 cheap/n' blow PSU
£13 DVD
£120 for Windows Pro OEM
£5 for nasty keyboard
£5 for mouse
That comes to £407, ok so ditch the Windows licence cos you don't mind dodgy, and forget mouse/keyboard, so that's £277 for which you have to build yourself, order all the bits, hope it works. For just £2 more you've got it prebuilt with a genuine licence for Windows Pro. But if you can build for less please share the info as I'm sure it would help many of us. But you can't find it for less when you factor in the Windows licence although I suspect you'll argue that no one pays for a Windows licence.

As an official Dell Vostro 3900 i5 deal poster I fully endorse this deal and give it my seal of approval

Cheers, having had my orders cancelled due to low stock on the other deals I thought I'd wait until my order was shipped before posting this one!

Curiously they have it also showing at the higher price http://www.cclonline.com/product/208125/1K6CF/Branded-Desktop-PCs/Dell-Vostro-3900-Mini-Tower-Intel-Core-i5-4460-4GB-1600MHz-DDR3-500GB-HDD-DVD-RW-Keyboard-Mouse-Windows-7-Pro-64-bit-Windows-8-1-License-included-1-Year-Warranty/SYS4563/

what would I need to do to make this a middle of the road gaming rig

I think the lower priced item is because it's a deal of the week

An additional 3 x 4 GB DDR3 1600Mhz single channel sticks of RAM, 500w + PSU with gold/platinum efficiency and ideally multiple rails, 24 inch IPS monitor with at least 1080p resolution/144hz, gaming mouse with at least 1800 dpi and 1000 polls per second, mechanical keyboard, 480GB SSD with ncache 2 or some sort of 256bit bus, a decent graphics card - either GTX 960 4GB or higher if NVidia or R9 range like 380 if Radeon with 4GB ram too. Personally I'd get a GTX960 and wait for the latest Polaris cards from Radeon like RX480 with either 4 or 8 GB, a Sonus or XFI range sound card, a slick gaming mousemat, Dolby 3D Surround 7.1 compatable headset, a Steam account with some games.

If you can wait for the new AMD cards due shortly, there should be one that will give you mid range performance without a power connector (assuming this machine's PSU doesn't have a PCIe connector). An additional 4 or 8 GB RAM would also help run things smoothly

This seems very OTT. This is how you would make it a high-end gaming PC, which if you were interested in doing, it wouldn't make much sense to buy a base rig like this.

If you want to play games on it, the main thing you should pick up is a graphics card (one that can cope with the fairly weak power supply such as a Nvidia 750ti). Some other upgrades could be additional RAM, addition of an SSD, and any peripherals you want to pick up; but all of these have only marginal impact on performance.