In the bleak mid-winter the mains water supply is at its coldest, possibly only around 3 degrees. I don`t know the power[KWH] of your shower but it may be struggling to heat the water to its usual temperature at its normal water flow output. If you reduce the water flow you may find it heats up more, but the only real solution is to go for more [say 9] KW, or less pressure until things heat up again around the Equinox.....22 March or thereabouts. It`s just a sad fact that in Winter things get cold.