Only lukewarm water from bath & shower yet sink is hot. Help please!?

House has 7 year old megaflo. Bath has separate thermostat fitted into wall. Can turn to cold. Ok water cold. But when set to hottest temp on thermostat then turn on separate wall fitted bath tap or shower it only comes out lukewarm water.
So all baths and showers are cold. Sink tap is hot as it should be.
Do I need to change shower/bath tap & thermostat tap? Is this possible when there is no sign of brand or make of taps? Or does changing taps mean I'd need to retile bathroom. Surely not! Please help,