So... for reasons too long and boring to go into, I got my local dealer change the thermostat last week.
Fortunately, due to a mix up on their part I only got charged 11/2 hrs labour instead of the normal 3 hrs. Thanks to them for honouring the original verbal quote!
And what a difference, Car now gets to normal temperature quicker (it never got to 90deg before - only about 80deg).
But more noticeable is the increase in performance. a lot more get up and go, especially low to midrange. A feeling of more grunt throughout.
I think its now going like a GTI ought to!
Fuel consumption seems to have improved as well. Too early to give an accurate figure, but it seems to be in the region of 2 - 3mpg.
I read on another thread about lack of 'go' when warming up and the suggestion that performance is 'restricted' until the engine is fully warm.
If that is the case, this would bear it out.
From my experience - if your engine is not running at 90 deg - bite the bullet and get the 'stat changed.