Hot water issue with combi boiler

[Lotte29]

I have a combi boiler, the hot water to my bath and taps upstairs is hot and stays hot, aswell as the hot tap in my downstairs bathroom, but in my kitchen the hot tap gets very very hot but it only lasts a couple of minutes then runs cold. I don’t know if it’s connected but the previous owner has what looks like a water filter system under the sink. Any advice please?
Thank you

 

[Jerry]

That shouldn't happen.

Going to need lots of pictures