Thanks Ralf.

I tried rebuilding the curve as your images show but the same problem still occurred(tried 20, 50 and 60 points).
However I did note that the distorted pipe seemed to be mainly at the middle top of the curve and deleted the middle point there.
Tried again and it worked! Don't know why.

Anyway, thanks for your help.

I would still like to know why the original curve didn't work however.

Cheers
Mal