With many thanks to Roger-Cheshire i now have a K&N air filter installed. More fiddly to fit than i imagined due to a hidden air-box lid screw - double joined fingers would help!
Result? Its difficult to determine exactly what improvement it gives - not that much i suspect, however the already keen to rev Sigma seems even more responsive - i suspect the gains are greater when revs are high.
Having followed Rogers advice by also fitting the lebrands side exit exhaust at the same time i must agree 100% that for less than £100 these small, easily reversed, additions add greatly to the car.
