I used to run the Super Unleaded all the time, when I owed a Puma 1.7 which was tuned & a Focus ST170 back then it was still < £1 a litre.
The only time I felt a major difference was with the Puma, when I chipped it on Super & went back to normal, the pick up out of junctions & on roundabout was dreadful.
The main thing super unleaded helps with is the carbon deposits on the back of the inlet valves, pottering around town is a major cause, taking the car on a high rev run or a road with tight corners & long strights, using the full revs, should help clear this, it will screw up your MPG if your trting to be Eco, but then if you don't your emissions go up.
I would agree with above posts, unless your using a a high powered engine, once every 3 tanks, would be a good idea.
Redex isn't cheap £7-8 a bottle & you put in ½ a bottle a tank, so couple of £'s at the pump, is cheaper option.