Yay! The plumber has connected up the hot water to the pipes under the floor, and it's all working! Not too much heat at the moment, because the concrete's not dry yet, but the floor is at a steady 15 degrees, and is warming the place gently. Lovely.
This is the installation. The warm water comes into the big red mixing chamber via the thick red pipes coming up from the floor, and via the left-hand pump. The pump to the right of the chamber takes the warm water and circulates it through the pipes under the floor, via the distributor on the far left, mixing it on the way with some of the cooler water returning from the floor so the whole thing doesn't get too hot. The electrics and electronics that manage it all are in the two smaller boxes attached to the wall.
The two pipes on the right, and the right-hand pump that all lead nowhere, are there ready for radiators to be installed in the rest of the house, later.
Soon I will enclose the whole lot in a little cupboard, but right now I'm just pleased it's all in and working.