The area of 454 ft2 is the gross area and includes the area of the fuselage. The distance from the wing leading edge over the radiators to the aft edge of the wing chord is ~14 ft, and the average width of the fuselage between the upper wing root lines is ~3.75 ft. If you subtract the area of the fuselage from the gross area you get ~402 ft2 for the net wing area. I am thinking that whoever ran the numbers for the manual (or whoever the person preparing the manual got the numbers from) added the flap areas to the main swing area in error. In all(?) the other British manuals I have seen, while the flap areas are listed, they are included the gross wing area - with the exception of aircraft fitted with Fowler or Fairey-Youngman type flaps, where they add the increase in wing area due to the rearward extension of the flaps.