I have had this before where the boiler is too small and to sufficent for the requirements put on it , with the cold weather at the mo you would have the heating on etc and the boiler would not cope well.
I would try to run the hot water tap slower so the the water can heat properly in the system before it reaches the bath.
I had cold-ish water for so long the thermostat on the boiler also changed so that it thought that was the temperature it was supposed to be heated to ??
Also try restarting the boiler there should be a button somewhere give that ago.
Find the manual for the boiler.