A double yellow line means no parking or waiting at any time; a single line means the restriction is in force at times shown on the plate. If it is a single line you need to check that they are parking in the permitted times. If they are, then there is not a lot you can do about it. We have the same problem only we don't have any lines. If people park opposite our drive we have major problems getting in or out. I consulted with the police who say that unless they are parked directly across our drive they are not causing an obstruction. If i am able to "catch" the people parking there, i politely ask them if they could move their car a little to left/right to enable us to use our drive. Most have obliged. It doesn't happen that often, but it is a pain when it happens, especially as our road is very long and there are plenty of other places to park so to choose to park at the narrowest post is a bit stupid.