How to flatten warped/bowed mirror back board?

Got a biggish G-Plan mirror. Has been stored upright for a while and back board has warped/bowed out at the back so when I hang it on the wall it projects out from the wall at the bottom. Have already tried storing it flat on the back board for a while to see if it would flatten. No go. Have also had "mirror mate" suggested by local joiner. Does anyone have any experience/comment/other suggestion on this, as the mirror can't be replaced if anything goes wrong, as it was end-of-line at the time.

Thanks