Sounds like the unit is simply not delivering enough heat. On high power the second element is switched on; if this is burned out (open-circuit) the high power light will be on, but the heater won't work, leaving you with only medium-power heat and flow. If you are competent to do so, check the continuity of the heaters at the tank electrodes after FULLY ISOLATING the unit. Or work your way through the fault finding chart on page 23 of the manual, which is on the Mira site at http://www.mirashowers.co.uk/onlinecatalog/pdf/mira_sport_max_1.pdf. Otherwise, it could be a loose connection or a burnt-out relay, but a loose connection would overheat very quickly and make itself known, and relays tend to fail closed (contacts burn and weld) so not so likely as an open-circuit element.
PlumbSpark
October 2009