Hello Howard,
I am not sure what answer you hope to get from Citroen? They have already tested the model and published the maximum towing limits, and they are satisfied that it meets the test criteria.
What you seem to be asking is how fast will it accelerate or what top speed will it maintain? Given that the quoted power is 70bhp, it is never going to win at santa pod, and with a substantial caravan with the aerodynamics of brick, you cannot expect it to set the world on fire, but it will get you from A to B but at a more leisurely pace than the solo car.
Just as an example of how aerodynamics really affect top speed. I used to tow with a Saab 9000, 125bhp. The outfit would easily pull a twin axle 1400Kg caravan and maintain 60mph on the motorway. Changing to a small box trailer and even when empty 350Kg) the car could not get the trailer above 50mph, simply because of the air resistance caused by the shape.
So Citroen could not possibly answer your question because the answer is uniquely down to the specific caravan you use.
I would suggest you check the Gross train Mass and axle loads for the car, and remember that they must include all the passengers and luggage.