Yep that'll do it! Oh and I've just figured out where 17mpg came from. That's using US gallons. 21mpg is using UK gallons.
When I do purely town driving in the ST I get less than 20mpg. With a mixture of town driving and some A road / motorway driving it goes up to about 25-27mpg. If I do a long run I can get 30+mpg.