On this green tank the big bit obviously runs round the front to the radiator.
With the little bit thats facing towards the flywheel side of the engine does it join onto:
That bit that looks like the same shape connection on the bottom of the thermostat housing??
Its clearly a rubber hose based on the only original pic that i have so hoping someone can clarify if im right in thinking as i cant find that hose atall on citroen diagrams