I'm not a heating engineer but have had my fair share of Baxi boiler issues through the years.
First things first.... With the current cold ambient temperatures the hot water will likely not get as hot anyway... With a smaller output boiler (24-28kW) and a flow rate of 9-10 litres per minute the water temperature will rise by a given amount. If the starting temperature of the inlet water is colder then the outlet water will also be colder, unless you slow down the flow rate.
As LG says, it could be the diverter valve..... Turn off your heating and allow the system to fully cool. Feel the temperature of the heating flow and return pipes to make sure it has fully cooled. With the heating still turned off, turn on the hot tap and let it run, slowly close the hot tap until you get the water as hot as possible. Does it get hot enough? If so it could just be the inlet water temperature being lower than normal or it could be the heat exchanger blocked. Leave it running for 15 minutes and then feel the flow and return central heating pipes. If they are hot then it's most likely the divert valve.
Most of the manuals for the Baxi boilers are available online and have a fault finding section in the back as well as part numbers should you need spares.
I'm not a heating engineering so feel free to ignore all the above!!