I too have the same engine and around town MPG varies quite a lot.
If I go into the City i get about 30mpg when the weather is warm, around 25mpg is it's cold.
Where I live is quite hilly and when it is warm I get around 26mpg and around 22mpg when it's cold.
I would say that I do a 50/50 split between Urban and Motorway driving and I average 33mpg.
18mpg sounds very low, in fact, I'm sure the 2.5L ST does better than that