shower thermostat problem?

the hot water from both showers in my home are now running slightly warm instead of hot. This happened to both showers on same day. We have a Homeserve contract & they sent out a Heating engineer who states memory card in both shower thermostat are broken, but then said we need a plumber!!! Its strange that both should go on same day, when gets lots of use, the other shower rarely used. Any ideas ???