Wondering why the living room rad was not getting hot, I remembered I'd turned off the one in the conservatory - common wall. Looking at that, I discover these two rads are in series, but there is the possibility to turn a screw in a pipe a 1/4-turn, opening a valve, and thereby short circuit the hot-water *around* the conservatory rad. This rad is still off and the living room one is now nice and toasty.
The bathroom also has a rad in series with the towel rail. Is there any rational reason for these sorts of arrangement?