Thanks all for the suggestions. Much appreciated. However, I wish I could say that they had filled me with confidence that there was an easy course of action. It does seem fraught for a non-local driver. After a bit more thought we have decided that we'll drive in and stay the night at the Willard. Spend some of those hotel points. We'll start from Winchester around Monday lunch time aiming to be downtown mid afternoon, visit the Vietnam memorial by foot and have a splendid great dinner somewhere. We'll maybe do a museum in the morning and head out to Frederick and on to Gettysburg. This is a Civil War Tour.

I'd advise anyone contemplating being on the DC roads that day to change their plans as the antipodeans hit town!

Again, many thanks!