Problem solved chaps.

It was actually two things, the Vsync limitation as discussed above, plus (the simplest of things!) my monitor refresh rate was set to 60Hz, vs its full capability of 144Hz. A bit like buying a Ferrari, then driving it around in second gear..

Once I removed those restrictions I got frame rates in the 140-195fps region. So I've gone back to HarryH's Vsync= 1/2 refresh rate, and also applied a frame rate limitation of 120fps. So far so good!

Thanks

Mike