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I assume that the gravity lensing difference between the two stars can easily be picked out because although they both throw a lot of light, they don't have the same spectrum, and probably not even the same redshift. You can then subtract out the closer star because you very carefully observed it when there was nothing significant behind it.

This is not gravitational lensing, but gravitational microlensing. From the BBC text I guess it's the planet's gravitational field modifying the star's light curve. The light curve is a plot of light intensity vs time. For a main sequence star, i.e., a "regular" one, say, Sol, there's no reason for the light curve to change over a small period of time (400 days or less, typical in microlensing events). Now, what puzzles me is an Earth sized planet's gravitational field isn't strong enough for the effect to be measurable.

You can look here and here if you are interested in more information about gravitational lenses. I couldn't find more details at Mount Stromlo Observatory's homepage.