Yes, today (10-4-10) & a couple of days ago...possibly the same one. Location - south west edge of Dartmoor.

I doubt it was the same one. Mine was in Marlow on that day, 23 March. It fluttered down the suburban street in front of me. I had never seen one and was mightily impressed by the colour. Are they common?

I don't think one would call them rare - but not common either! However I'm no expert! To my great shame, I used to catch a lot of butterflies when I was a child - there were more about then (the 60's).
I googled the Brimstones food plant - Purging Buckthorn and Alder Buckthorn, so I imagine these are growing in your area & mine.
I haven't seen any more Brimstones since - but the weather has been quite cold. I have seen an Orange Tip in my garden and a brown butterfly (possibly Meadow Brown - (I wasn't paying attention) in the lane.