I am flying into Dulles from England in September. Can anyone give me a rough clue of the cab fare from the airport into downtown DC.

$50 - more or less. The cabs are metered, but $50 is a good estimate for getting to downtown DC.

There are other options, such as the SuperShuttle, which cost less. For 2 or more people sharing transportation, the cabs could be a better option, since the SuperShuttle charges per person.

In addition, depending upon the amount of luggage you have and your fatigue factor, there is public transportation.

Let me know if you are interested in the 2 options for public transportation, and I'll detail them.

You picked a good time to visit. Here is info about ground transport from IAD:

http://www.metwashairports.com/Dulles/ground.htm

I was there this month and the fare on the shuttle was $41 for 3 people one way. It would probably be a better deal on a return fare.
If you click on my name I have done a short trip report which might interest you.

Thank you each, I am humbled by your assistance. Travel Fan, any info would be greatly appreciated.
Seahatch, your trip report was a great help because I am busy trawling through all the potential sights to cherry pick what I need to experience in the time I have in DC. Once again, thank you.