Ariston Microgenus 23 combi boiler

My boiler recently developed a fault where I had central heating but
no domestic hot water (stone cold). I was advised to fit a new
diverter valve which I did.

The problem I have now is that I still have central heating but the
domestic hot water only runs hot for 2 to 3 minutes then runs luke
warm although the boiler is still fired up.

Does anyone have any suggestions what is causing this to happen?