The measurements, as graphed above, show a steadily increase in price since early September, just when Mango-fever was picking up. The jump in price is also fairly significant, with the 7 Pro going from an average of £240 to £350 in just 30 days.
The market can be a volatile place, but the correlation between the Mango publicity (and confirmation that old devices would get the new software) is irresistible. There's a lot of people interested in Windows Phone, and maybe the Mango news was enough to push them to look for a cheaper way to try the new OS than a two year contract?
More details, and pretty graphs, at WP Central.