I am not exactly sure witch model you have however you can buy the button as a spare
http://www.showerdoc.com/shower-spares/mira/MIRA-417-84-Mira-Start-stop-Button-Sport-9kw-7-5kw-old-10-8kw---Elite-Ii-st-(oval)-417-84
or you may need the complete cover assembly.
http://www.showerdoc.com/shower-spares/mira/MIRA-416-60-Mira-Front-Cover-Assembly-(oval-Button)-416-60