Sounds like a problem with the temperature spindle - there is a locking washer on the spindle to stop the temperature control knob from rotating 360degrees. The locking washer engages with a stopper on the spindle to prevent the temperature control knob from spinning a full circle. Either the spindle or the washer could be damaged. Mira does a repair kit but it is around £100 inc VAT.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.