Its was the motorized valve when I had the same symptoms ..... have you something like this on the pipework going into the hot water tank?
https://www.screwfix.com/p/honeywell-home-40003916-001-2-port-v4043h-replacement-powerhead/326fv
Was an easy fix (no plumber required) in my case
## Before I got my hands on the replacement part I remember removing it from the pipework so I could manually open the valve to allow hot water into the tank but you'll need to be mindful about closing it again at some point when the water is hot enough (take picture of the close position before doing it)
https://www.screwfix.com/p/honeywell-home-40003916-001-2-port-v4043h-replacement-powerhead/326fv
Was an easy fix (no plumber required) in my case
## Before I got my hands on the replacement part I remember removing it from the pipework so I could manually open the valve to allow hot water into the tank but you'll need to be mindful about closing it again at some point when the water is hot enough (take picture of the close position before doing it)