I may be a good idea to recommision the shower. Instructions on how to do this are contained in a video on Mira's excellent website http://www.mirashowers.co.uk/support/videos.htm
If this doesn't work you could always book one of their Engineers by ringing their Service Department on 0844 571 5000
Rob
July 2010
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howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.