Looking into the recommendations given here a little further (thanks for the replies) so far the 3700x really does look to be the best VFM currently, and seeing the results of the 3300X's "1 CCX" design in reviews released last night makes me think whatever Zen 3 brings will be yet another big step, if my 2600k lasts that long.
I also looked at Intel's and the 10700k looked interesting as a 9900k "refresh" but for far less cash (still £400-ish?) but with it's large increase in power requirements I'd be worried about how hot it runs even with "die thinning". However there aren't any reviews out yet so I'll wait and see how they turn out.
It's been an eye opener to catch up on the recent advances, I think the last time I looked seriously at an upgrade was around 2014-ish and there wasn't any point at all so I stopped keeping up with the CPU side of Tech and only bothered with GPU and the past 3-4 years have been a CPU game-changer which I'd completely missed.
I'm exactly the same, spent a fortune on a high end X79 system and it has served me well. The only reason I am thinking upgrade time is because I bought a new 34 inch monitor.
I'm tempted just to upgrade the gfx card and wait for Zen 4, or the other option is to go all in on top spec Zen 3 and miss Zen 4 completely.
I always tell myself to upgrade gradually, but once I build a system, I can never be bothered taking it apart until I need a full system upgrade.