ChrisJ wrote:
Oh dear!
The pipes went down and were tested as not leaking.
The screed went down dry and no leaks showed up.
The floorsheets went down and then the ply and ceramic tiles.
And then the plumber came to connect the manifold to the boiler.

and we discovered we have a leak somewhere under the floor which
wasn't there when the screed went down.

Any suggesstions welcome as we really don't want to start all
over again!

Bad luck, that must be so annoying.

Have you considered abandoning the UFH and installing conventional rads?