Carnut and Hondast, thank you for your comments.
Sometimes locking the puzzle works but just occasionally it causes a problem.
Two points for a difficult puzzle where there are two distinct elements (eg - chassis & coachwork) can work well and keeps the puzzle of interest to those looking for the extra point.
Anyway points are not everything (I've been stuck on the same number for a while now) - I just enjoy the challenge (when I get the chance!)