Since fillting new toilet to existing 4" elbow outlet which goes into the downcomer, it freezes in the 4" elbow outside. When unfrozen cistern empties fast implying there is no blockage in elbow. Previous toilet did not do that for 30 years. There should be no water lying in the elbow to freeze. Toilet is connected to elbow with flexible 4" pipe. Is there a level issue eg toilet level too low or fall on elbow is too horizontal (difficult to see on current)?
Either standing water in the pipe is not draining away because of a blockage or incorrect fall and eventually freezes, or the cistern is constantly overflowing which means the ice builds up over a period of time eventually blocking the pipe.