I can mirror your part no issues.............
Firstly, you won't be able to thicken this by 7mm because of some of the geometry of it. Some of the curves, arcs, corners, spaces are too small to facilitate a 7mm thickening. For example, you've barely got 1.5mm gap in the cavity that overlooks the gauges so that may as well be solid. Also, you have done really well with this as a new SolidWorks user but you really should do the tutorials and learn about fully defining ALL of your sketches once they are complete, or at least try to.
Thank you dave! After trying harder I actually got it to mirror too!
" For example, you've barely got 1.5mm gap in the cavity that overlooks the gauges so that may as well be solid. "
Oh, I see... so the best option would be make them become solid? Will try to do that, thanks!
What are the advantages of defining the sketches?