Nice showers for the price and at 10 years you havent done bad. It sounds as though the thermostat is breaking down and sticking. Check to see if you can replace the part, if not then its a replacement shower. The distance between the pipe connections could be the same and possibly it may be a case of shutting off the hot and cold supplies and undoing the nuts and putting the new unit on (plus washers). Check with the plumbers merchant and see.
plumbo
December 2010