I rode this track around this reservoir the other day (and further west until it joins a paved road). I expected, judging from the colouring on cycle.travel, for it to be mostly a smooth cycle path (only a short section in the middle was marked as unpaved). Not so! It's rough the whole way, and pretty hard work in many places. I have since updated OSM to change the surface type.
OSM may still need more work perhaps Richard could check?