I’ve been to my daughter’s again this morning and taken a few more pics and felt the temperature of the flow and return on the underfloor heating manifold. The 15 mm supply pipe is very hot until it joins the valve arrowed yellow which links with the return flow on the manifold after that it is only lukewarm and the temperature gauge shows around 28 degrees. I am not a heating engineer but is this a mixing valve and would turning the nut increase the ratio of new hot water to the return water from the manifold ? Her floors are definitely warmer but not as warm as they should be which makes sense if the water flowing from the manifold is not hot enough.