Well... here's nvidia's answer:
"Hello,
Our lab was able to replicate the stuttering issue but we see the same stuttering when 260.99 as well. We even tested with AMD/ATI GPU and see the same stuttering. Either we're not seeing the issue you're reporting or we're using a different version of the game. To confirm, you don't see this stuttering when you go back to 260.99?
Best regards,
Ray"
I haven't rolled back my drivers in a long time (and tbh I can't be bothered messing up all my custom settings, custom resolutions etc) so I may be wrong about 260.99 being the last non faulty drivers (maybe it was 258.96 and those before that were fine?) but they say they see the stuttering on AMD/ATI GPUs as well, wtf is this? They must be talking about a different issue... or M&B players are all noobs that have been playing a stuttering game for 2 years or so? No, I obviously don't believe that.
And I have seen the game running smoothly on my mates PC recently (oddly enough he has a nvidia 560 and the latest drivers and the same CPU/MOBO/RAM/PSU/OS/Monitor etc as me)