2004 Nissan Micra Orange light

Hi, An orange light illuminated on my elderly mothers Micra a few months ago. She took it to a local garage who put it on a diagnostic machine and charged her £75 to tell her there was nothing wrong with the car and they could turn the light off for her. Since then her m.p.g has gone down by 50%. Could this be something to do with the light that came on which does still happen now & again. Has anyone had a similar problem or know how to repair this or whether it will cost very much. I live in another country so I'm limited to what I can do. If she had an idea what was wrong she could take it to a local garage & not get taken for a ride.
Thanks