We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember BlueJohn
saying something like:

On top of the heating boards is 17mm plywood, some thin underlay
(recommended by the manufacturers for UFH) and then B&Q click-to-fit
wooden boards (also said on the pack that they would be OK for UFH).

Probably too much insulation on top of the pipes, in the form of a
floor, and I suspect there were no heat spreader plates in the system.
If the pipes were fitted straight into a moulded foam carrier, the heat
has to rise just above the pipes and can't spread out very much. This is
also assuming the foam pipe trays were well insulated underneath, as
heat robbing undersides might be contributing to the problem. How thick
are these foam trays, and were they fitted straight on top of concrete,
or in a joist gap and if a joist gap, was the gap filled with rockwool?
How much foam is under the pipe when it is fitted in the tray?