How can i get hot water through my Aspen white water mixer shower ?

Unable to get hot water through Aspen mixer shower. Have stripped down mixer unit to reveal 'thermo capsule' This was seized, so corrected and reassembled. Flow rate is now much improved but water temp is cool with mixer fully over to the hot position. Stripped down again have got a reasonable flow of hot water through unit without mixer value in place. But again when mixer unit is in place water temp is cool. - Should be ok in mid summer but at the moment I'm just getting ear ache from the women in our house....