Absolutely agree. The rest of the preparation may well have been to the same poor standard.

Try a Citroen dealer if you want a used C5, and see if you can find one that they've taken in PX for a new Citroen. This means that the previous owner will have liked it because it gave good service.

I agree with the posts above. However, you could use it as a lever to sweeten the deal in your favour.

You could tell them the deal's off unless they sort it; and then start haggling for a big discount, based on the fact that you will have to make good the dealer's failed promise and that the damage obviously requires a new bumper (you could price up a replacement bumper with a call to a Citroen dealer, plus labour, so you have an idea of costs).

If they want to move the car on quickly, they might cave in. Then you could Chipsaway or similar to make it good

Does the car have a good service history etc otherwise? If it's good otherwise, you could get a very good deal with a bit of firm negotiation based on the replacement costs of the bumper.