Hoftraveler, thank you so much.
It seems the way to go is to do Mount Vernon and Alexandria in the same day - ie. the estate in the morning and spend the afternoon and dinner in Alexandria, then metro it back to the hotel.
I've had a look at bus and train schedules as I originally thought we could get the train to Huntington, then a bus to the estate. Then bus back to the station and travel the couple of stops back towards DC to Alexandria (I've momentarily forgotten the name of the station). But the buses and trains aren't timed well and the bus is about 1/2 hour. I don't want to spend half the day on transport.
Would it be possible to go to Alexandria by metro and catch a bus from there to Mount Vernon - as this way it would drop us back in Alexandria in the afternoon? Either by public transport or an organised tour...
Also, how early does it get dark at the beginning of Jan? Of course, when wandering about, we'd actually like to SEE things. :)
I found a walking tour at dc.about.com/cs/sightseeing/a/oldtown.htm
which I thought we could follow. Comments would be most welcome.
Thanks in advance for your help. This forum is fantastic and I also try to repay my debt, by helping out those travelling to Australia!!