Welcome Bezoomy !
I built the Airfix Sea King a couple years ago. Yes - there are extra "mystery parts". I think one of them is a sonar dipping probe. I didn't have any problem with the drive shaft except that the fit is loose. So if the rotors were really powered - they'd wobble alot.
I only had 2 problems - one was a lack of experience with painting yellow (not a fault of the kit) I'd recommend priming with a white primer first. The only problem I had with the kit was the clear front windscreen. It was warped so that it stuck out on either side quite badly. My solution was a bit odd - but it turned out well (I think). I used a hack saw blade to cut the windshield into two parts. This removed about a 3/32" (~2 mm) width from the piece - just enough to correct the width increase from warpage. The trick is making the cut in a position that will not be noticed. You dont want to split the thing right down the middle - then you'd see the split ! I made my cut just inside the frame of the row of windows that run down the center. This made the center section of window glass slightly narrower than it was before the cut, but it kept everything symmetrical and it made the cut in a place where it is now unseen. After trimming and sanding/polishing the cut edges, I just used clear white glue to join the two ~halves of the glass part.
I just finished the Revell Germany Sea King about two months ago. It is a more highly detailed kit. The parts fit was excellent, but again - the glass windshield piece was just a little wide. I imagine that molding a piece like that is probably pretty difficult.
I found both of the kits to be very enjoyable builds.
Chris