Do you need Pro or how about 7 Home Premium?
http://www.scan.co.uk/products/microsoft-windows-7-home-premium-64-bit-sp1-operating-system-single-oei-dvd

Thanks. I suppose I don't need it but is XP Mode on the Pro version likely to be useful?

I've seen a couple of OEM copies on eBay for around £30 (from decommissioned systems) and, from seller feedback, folks seem happy. 

Intell 6600 or 6600K, if you want to OC
H170 Chipset Motherboard equals no OC.
Z170 Chipset Motherboard will give all the features & Raid and OC Ability if you wan't it, plus 2 gpu's at 8x per slot
(note for motherboards i always go Matx and go for a smaller case)
8GB or 16GB 2666 Memory, i always go for Corsair but its preference
250GB Crucial BX100 / Samsung 850 EVO
1TB+ Drive of your choice, i only use SSD's these days, everything loads faster.
Radeon R9 380 GPU, i always go for MSI Dual Fan ones with 4GB Memory. If you can push another £100 look at the Nvidia 970 which is the best value/power for money.

I priced it with a:

Fractal Design Case, this is my preference for an MATX setup, but there are 100s of options.
EVGA Supernova 650W 80+ Gold Modular PSU. Semi Modular & Gold rated for under £70 is a bargain, this is what i'll buy next, i have been using Corsair but have had a faulty one recently.

OS your choice.

Screens, i always go Dell Ultra for the quality/ and unrivalled warranty, but they are not in the £800 unfortunately!

Thanks for that. Not looking to upgrade the monitor just yet. Big Dell thingy. Golden. And glorious enough for my needs. 

H170 Chipset Motherboard equals no OC

Do you mean you can't or you won't need to?

I might not have made clear: I'm looking to buy off the shelf.  Just don't have the time to do a build these days.  Are Novatech well regarded?

I'd just like to point out i've been using a standard cpu cooler for years and they are not so bad, i have tinnitus and its quiet enough not to bother me.

Intell 6600 or 6600K, if you want to OC
H170 Chipset Motherboard equals no OC.
Z170 Chipset Motherboard will give all the features & Raid and OC Ability if you wan't it, plus 2 gpu's at 8x per slot
(note for motherboards i always go Matx and go for a smaller case)
8GB or 16GB 2666 Memory, i always go for Corsair but its preference
250GB Crucial BX100 / Samsung 850 EVO
1TB+ Drive of your choice, i only use SSD's these days, everything loads faster.
Radeon R9 380 GPU, i always go for MSI Dual Fan ones with 4GB Memory. If you can push another £100 look at the Nvidia 970 which is the best value/power for money.

I priced it with a:

Fractal Design Case, this is my preference for an MATX setup, but there are 100s of options.
EVGA Supernova 650W 80+ Gold Modular PSU. Semi Modular & Gold rated for under £70 is a bargain, this is what i'll buy next, i have been using Corsair but have had a faulty one recently.

OS your choice.

Screens, i always go Dell Ultra for the quality/ and unrivalled warranty, but they are not in the £800 unfortunately!

Was the above priced on Ebuyer?

What brand is good for motherboards? I'm tempted to stick with Asus.  I think I'm just gonna get a motherboard, graphics, RAM and memory for now and 'bung it in'... seems a waste of money to get a whole new rig when everything else is still good.  I have a mid tower Antec ATX case and a 620w Corsair modular PSU .

This has an i7 at 6700k but only Kingston memory: http://www.novatech.co.uk/motherboardbundles/view/mbb-670016.html

A Gigabyte mobo and Kingston memory... worth a punt?

Well, well... so much for a cheap fix:

Intel Core i7 6700k Skylake Processor
16GB Kingston DDR4 (2x8GB) 2666Mhz
GIGABYTE GA-Z170XP-SLI Intel Z170 (Socket 1151) ATX Motherboard
Arctic Freezer 7 Pro Cooler
MSI GeForce GTX 970 GAMING TWIN FROZR V OC 4GB GDDR5
Corsair RM Series RM750 ATX Power Supply 750 Watt 80 PLUS Gold Certified Fully Modular PSU
1 x Samsung 850 Evo Basic 500GB Solid State Hard Drive 2.5" Basic Kit with Data Migration Software - Retail
Microsoft Windows 7 Professional 64-bit

That should keep me going for a bit, right? 

The 6700k was part of a bundle (and the only i7 Skylakes they do apparently) so went for it anyhoo. 

Thanks all for the advice. Cookies/virtual beers on the way. 

Going for assembly tonight - wish me luck!