It might be worth attaching a copy of dh's passport to the letter then straightforward the signature can be matched to the letter and should be pretty sufficient. I went to Germany recently with my son and because my son does not have my surname, I was stopped by the immigration and questioned. They did not understand why I was not using my married name when my son was ???? I showed them my maiden name on his birth certificate but they warned me that next time it would not be accepted, you can't have too much information, have a nice trip.