OK... quick report on first findings since cleaning...
My daily trip to work is 33 miles, half motorway. I usually return 46-48mpg, depending on how many traffic lights I get stuck at. My best ever was 49.9, that was a very clear run.
This morning having cleaned the MAP, I got 49.9 after being stopped at every single traffic light!
To back this up, the mpg reading is usually on 49-50 as I leave the motorway. This morning it was 54. And the first three or four miles of motorway is all uphill, and the instant mpg reading is usually high thirties, this morning it didn't drop below 45.
So initial results are positive, looking like a 5-10% boost in mpg. I'll keep you posted. Shadeyman's video is a godsend!