You could individually attach the vertices. Under the "polygons" menu set, hit edit mesh and look for the "Merge Vertex Tool". After you have selected the tool hit the "Snap to points" button (button 3 of 5 of the red magnets at top, along the menu set). Shouldn't take more than a couple of minutes! There's probably a much quicker way, hope this helps.
Thanks Dayle, where did you disappear to? I did this method with my previous attempt at mirroring and it was tedious but yes it got it done, I was wondering if there was an alternative, better way. Thankyou for your help :)