We had the same letter, wife was in hospital so never returned it (wife is irish, kids born here but have my english surname)
the child benefit got stopped from Feb 2010, filled in there letter sent it off called a few times and told we need to wait 3-4 months to get it again... 3 months gone so expected it 1st of next month, we have a new letter they want more info now! wife went straight into the office to speak to some one show the kids passports hers etc but they say it will take 3 or 4 months again.
any one else having this problem it means my children have had no benefit this year for what will be 8 months if they sort it out, kids are at school here it all seems a bit mad. I am sure we are going to get a letter in 4 months time that says we cannot claim a back date for the last 8 months or something? and then it will all make sence... by the way none of her 4 sisters got these letters, so surnames seem to be the key!