New PC for ~£900 ($1.4k)

XFX make awful GPUs - do not buy one (their PSUs are very good though).
Go with Sapphire for AMD cards.

Also, if you're reusing RAM from your old PC, you can't "mix and match" it with the 8gb Corsair module on your list. You should only use identical modules. I'd say just buy a new 2x4gb kit (better than 1x8gb), and keep the RAM from your old PC for spares.
I'd recommend this RAM. It's going cheap, and performs great (I have it) - http://uk.pcpartpicker.com/part/patriot-memory-pv38g160c9k

Great choice on the monitor and keyboard - I have the exact same ones, and I can tell you that they're both superb.
The X4 is a very good keyboard, especially considering it's price. Even though it's not a mechanical keyboard, it's very good for a rubber-dome.
That ASUS monitor is great too - cheap, but the quality of the image is sterling.

Also, if you aren't planning on SLI/Crossfire, it might be better to go M-ATX, as M-ATX motherboards are cheaper than their ATX counterparts.

Instead of that 600W Corsair PSU, get the XFX 550W PSU - http://uk.pcpartpicker.com/part/xfx-power-supply-p1550sxxb9
- it's cheaper than the Corsair, but it's a very high quality PSU. 550W is more than enough for that PC anyway

As for the mouse, the Logitech G400 is good, but I'd recommend a Roccat Kova+ - I have one, and it's a superb mouse. It's cheaper than the G400 too. However, it may not be to your taste, as it has LEDs (which can be turned off) and quite a "futuristic" design - I know some people prefer minimalistic mice.
- http://uk.pcpartpicker.com/part/roccat-mouse-roc11520

Also, you may want to go for a 1tb HDD - 500gb won't last you very long, if you're anything like me (I've half-filled my 1.5tb HDD which I bought only 5 months ago). I'd recommend Western Digital's Caviar Blue HDDs over Seagate's Barracudas, although Barracudas are still very good HDDs, and reasonably priced - Western Digital HDDs are more expensive, but are very well made.
I'd recommend this - http://uk.pcpartpicker.com/part/western-digital-internal-hard-drive-wd10ezex
- It's only £13 more for twice the storage of the Barracuda you chose, and has a much larger cache - a wise purchase.

Finally, you don't need a 120hz monitor. A hd 7850 wouldn't be able to fully utilize more than 60hz (except in TF2 - even then, you probably won't notice much difference between 60fps and 120fps). 120hz is only truly justifiable with cards such as the GTX Titan, GTX 690, GTX 780, HD 7990 (etc.) or a powerful Crossfire/SLI set-up. Besides, I can guarantee you that that monitor is great.

In summary, here are the changes that I would recommend, but do not feel obliged to follow them suit:
- http://uk.pcpartpicker.com/part/roccat-mouse-roc11520
- http://uk.pcpartpicker.com/part/xfx-power-supply-p1550sxxb9
- http://uk.pcpartpicker.com/part/western-digital-internal-hard-drive-wd10ezex
- http://uk.pcpartpicker.com/part/patriot-memory-pv38g160c9k

EDIT: Also (and this is a case of personal preference - I do not expect you to agree with this), I'd get an Ivy Bridge CPU over a Haswell, for a handful of reasons that probably wouldn't apply to you.

Hope this helps