SO I finally got some time to fit the new knock sensor (have a 6 month old baby so time in short supply). I also replaced the thermostat as the car wasn't getting warm.
The car is much better although still hesitates on occasions, this time around 6k revs. My feeling is that it has poor quality fuel in it, it just feels sluggish. I'm going to run through the remaining 1/4 of a tank and put a tankful of decent fuel in it.
Think I'll also change the plugs so I know they're not to blame.
Cant think of anything else it could be?!