you need to open the manhole where the drain goes to - then put a piece of wood in the channel over the outlet - flush the wc until the boss floats down and stops against the wood . Pull iit out.
Well you could do what I had to in the 70's as an apprentice - Rawlplug tool+ club hammer then a go with a " hammer drill " up a ladder with 240v. extension lead snaking up the ladder :eek:
worth a try turning them off - take a chance . Or try turning the pump off and seeing what happens - the thermosyphon( gravity) circulation may do something on it's own .
The original cylinder probably had an annulus inside it ( love that word) The large opposite tappings give it away. So a new one would need at least a 28mm coil inside. Possibly controlled by a Cyltrol valve on the return - if they even still make them.