Before you go changing the lambda sensor (which is quite possibly faulty, i wont deny that) i would personally check condition of the plugs and air filter, easier to do a good service first and compare your mpg. Lambda sensors on the micra exhaust manifolds DO NOT come out very easily....trust me, i have thrown manifolds away because they just will not budge even with all the equipment i have at my disposal!!! Do the obvious things first, dont get caught out and throw money and time at something before doing the basics. A dirty air filter, poor sparks plugs with have a big impact on mpg and, to a certain degree, the grade/quality of oil in the engine will too. Check for little things like binding brakes, incorrect tyre pressures etc
I would not pay too much heed to mpg figures given in owners books either. These are calculated in ideal world conditions and generally are a load of b*llocks. Automatic gearboxes are a killer on small cc engines no matter how good those engines are (and the cg engines would certainly be the best of the bunch that way)