Sorry for late reply, and thanks for all the responses!
Having read through them all, there seem to be mixed responses but the general consensus is that permanent-AMD is off the table, 4790k is sufficient for gaming right now and for a couple years, 5820k is good if money is no problem, and Broadwell K is not worth the wait.
So, an update from OP as it were; I want to keep my budget around £400, and have arrived at three options ranging from "safe idea" to "could be stupid", respectively:
Intel i7-4790K
Stock cooler or current cooler (Arctic Cooling Freezer Xtreme, I saved the Intel parts)
ASRock FATAL1TY Z97X KILLER
OR
Intel i5-4690K
Corsair H100i (for overclock)
ASRock FATAL1TY Z97X KILLER
OR
AMD-8350
Corsair H100i or current cooler (Arctic Cooling Freezer Xtreme)
Current motherboard (ASRock 970 Pro3 R2.0)
ASRock 970 Pro3 R2.0, so not amazing afaik.
I've heard some not-so-good things about ASRock's Mobo's. Try Gigabyte or ASUS if you can fit it in.
Also, as has already been said - don't play the waiting game, you'll be waiting forever for the 'next best thing' otherwise!

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