Re: 18 MPG??? Is that right?
You should be getting around 26mpg+ at least in town. Mind you 5 miles travelling each way hardly gives enough time for the engine oil to do it's job properly.
I would suggest checking your oil level first and top up if required, check your tyre pressures are ok. They should be around 33psi. Then fill the car full of petrol and run it down to, say, 1/8 of a tank, and then filling it back top full to properly calculate MPG. If you are getting the info from your trip computer I wouldn't trust it's accuracy.