you need to find and old skool tuner as most mechanics these days are just fitters, you can alter the mixture if you can use the exhaust gas anylyser at the mot station (or elsewhere) there is a small screw between the elbow on the airbox and the metering head, if you look from above it you will either see the tiny rubber bung that should be there but usually isn't or a little 6mm hole, in that hole is an allen key/torx key (can't remember which) screw that you turn to adjust the mixture, that's pretty much all they would do on a tuneup, or you could go for a rolling road tune, but ask them if they know how to alter the mixture first just incase you pay for the tuneup but they don't really do much to the car
