Sorry, can't find the original thread.
The faulty part was a rubber expansion hose positioned right next to the heat exchanger. (Who do Vaillant employ as engineering designers? The lavatory cleaners?)
The replacement part is a telescoping pair of copper pipes, with O rings completing the join.
But, now that it is working, once the system has come up to temperature and slips into anti-cycling mode, there is a strong resonance vibration that is audible right through the house.
Is this to be expected?