how difficult is it to put a toilet into a older flat?

unless we can see the flat it is hard to tell where you will put it,but a good plumber will be able to sort it out ,if its to far from soil stack or drain to run in 100mm soil pipe you could install a saniflow.

Hi it totally depends on if the soil pipe could physically be taken into the location of the new toilet. This is a 4 inch pipe that needs to fall all the way back from the toilet to the main soil stack. If this is not possible you could fit a macerator which cuts everything up and then pumps the waste product out into the soil pipe. The size of pipe for this option can be as small as 22mm and can be pumped vertically depending on which macerator you choose. Hope this helps.

You will need to connect the toilet to the soil stack (4") pipe with enough fall to allow waist water to be dispersed if it is not near soil pipe you can use a masorator which will chop up waist from toilet and will only need a smaller pipe going back to the soil pipe