Do check out this excellently-researched blog post from the think-tank Open Europe. It details their finding that, as of yesterday, the pound has fallen by more over Brown's premiership than it did on Black Wednesday. And there could be worse to come. As Open Europe put it:
"If the current trend continues the pound will be down about 18-20% by Brown’s first anniversary. Brown's Black Wednesday may be happening more slowly compared to Lamont's - but it could be bigger."
Here's their neat graphic, with the Brown era represented by the pink line and the Lamont era by the blue line: