As a result of this thread, i just went to Toolstation and got 2L of meths and added it to my tank. I have also been experiencing an intermittent misfire and down on power even when hot and to be honest, i should have thought of it sooner.
I had half a gallon in a can in the back anyway from my last trip to fill up (i'd left things a bit late!
) so i added the meths to that and then added all of it to the tank.
Fired up after turning a bit more than usual but it was a hot day and as i was thinking i may have to try again, it caught and idled smoother than it has for a long time.
Drove out of the car park and put my foot down to pull away and surprised myself - it went better than it has for a long time too!
Overall a vast improvement! Of course, the acid test will be when it's sat overnight and how it performs from cold tomorrow but so far so good!