Oh, sure that's a catch of the day! I love those RSW-16's, and have always wanted a folding one. Not that I need it, of course.
Seat post is 28.6 mm IIRC, same as a Twenty. Just get a cheap aluminum one, they're better and longer than the original Raleigh one.
Does the rack have a spring-loaded catch on it? These bikes came with a tartan shopping bag that clamped to the rack in a most ingenious fashion.
Yep, I believe the rack is original, with the spring loaded rear section. Thanks for the seat post info. Tires and seat post are the next purchases.