You should not have sent it to them as they will say they never recieved it. Even if it was sent via recorded delivery they eould have said the the packaging was broken and item was not inside. If you refused to send it to them and threatened to take it to the media/local newspaper and seek legal advice they would have sent a rep from their company to come and see it and offer some form of compensation straight away as they dont like negative media attention.
If you called trading standards before contacting muller they would have advised you to do the same thing I said.
My mate got £1200 compensation from subway after he found a piece of plastic in his sandwich
Pics?