I would say I was drinking the beer at a party, so never heard any rattling etc, and the metal piece ended up in my mouth and I nearly choked!
You have sent an image, and to me it clearly looks like a nozzle of some kind used to dispense the beer into the bottle.
Tell them you will be contacting the media if you do not get a suitable response. You are willing to send the item back once an agreement has been made.
That is essentially Fraud, you were both well aware there was something in the container (as well as beer), so lying to them saying you nearly choked whilst at a party is both a lie and a fraudulent claim on your behalf, a better option would be to have them collect it and get the issue sorted, perhaps this will stop someone "Genuinely" choking on an item in the future, rather than you lying to them to get free beer!.
Just to be clear I have never lied to anybody throughout this, how could My mate possibly not notice a 3" metal tube in his beer?
I have contacted citizens advice and somebody from environmental health is coming for a meeting on Monday. I have also given Carlsberg all of the information required to start the investigation.
As for the taste of the beer, bear in mind it would'nt taste half as bad if it was free!
I will keep you posted.