Hi Pete,
I have 3 BSA's with that type of nacelle and I had to go to the garage to check them out

. Yours is quite correct. The top half of the trouser leg is flat and the bottom half is round. I think the problem is within the photograph as we very rarely see a picture of the nacelle from that angle.
Welcome to the forum. Have a Happy Christmas and a great 2007.
Beezageezauk.