Its the pipework to one of the UFH manifolds. Someone came along and thought putting a full bypassing tee between the flow and return?
Was he just looking to impress he could afford a pressfit machine? The original callout was to tackle an issue with the hot water not getting sufficiently hot. Who ever did this did not resolve the issue. Subsequently it turned out to be a bit of air and the boiler cutting out when on low gas due to incorrect fuel/air ratio.
Thank you Chalked, you are right and since the introduction of this tee the heating is not functioning well anymore as all the UFH pump is doing is recirculating the water within the loops. I can see how the misinterpretation / lack of understanding of close coupling may have led to this now. There is a header on this system already by way of pipework albeit not a low loss one.