How can I politely tell my parents they need to learn table manners?

I don't come from a rude or redneck family or anything like that but there's this little get together coming up. My girlfriends family recently invited my parents over to dinner. They are a very proper English family. Unfortunatly my parents aren't so proper. They chew with there mouth open and while talking. It's kind of disgusting. I don't want to be rude to them so how can I just tell them to use better table manners? Especially while at my girlfriends house.