SSAO Screen Space Ambient Occlusion / Obscurance on ios

Can anyone help out with a working shader to create an SSAO effect on IOS please? I'm using the latest Unity Pro version with the latest iPad (iPad Air). The SSAO script in image effects deosn't seem to work at all but the Ambient Obscurance script works to some extent. It works but none of the smoothing options take effect, so it appears grainy (fine in the editor but grainy on iPad). Also, using this script causes a huge drop in fps even in a very simple scene (1 plane and 1 cube), from 30+ fps down to around 12 fps.

Has anyone had a similar experience and managed to find a workable solution to this?

Many thanks!