Hello,
Assuming that there is not a problem with the cable from the isolator switch to the shower, it sounds like the PCB is faulty. You can have this replaced. If you would like to call us on 0845 126 4141 we can help further, or alternatively, visit our web site (link attached) where you can see a diagram of the shower and order the part online if you wish.
http://www.showerspares.com/products/spare_parts/mira_sport_max
NATIONAL SHOWER SPARES
July 2008