If it really worries you, clean your TVA, replace the M.A.P sensor and the Lambda sensors and then get a new exhaust, a lot of power can be lost because of old exhausts, particularly the CAT which is unfortunately the most expensive part.
To do all this will cost you around £650 but you will restore some of that MPG, not all of it however.
Use good quality 95 ron fuel too, like BP or Shell.