Thanks to everyone who helped solve this problem - one that has been bothering me for some time.
As I suspected, it now seems embarrassingly simple. As Gaieus said, my problem was drawing the single line curve through 3D space.
Eric [Boofredlay] - a simple, clear and efficient method that I will now be able to use on other models. Very much appreciated. Shouldn't this go on the Tutorial Forum?
Jim - yours is another simple method that would work because you could vary the size and shape of the two curves to avoid other objects that might be in the way. Many thanks.
Shaun - I appreciate your efforts. You got a lot further than I did. You actually had curves which is more than I could manage.
Howard - I intended to use this ruby plug-in or one like it but had to draw the curve first. Thanks for pointing me in the right direction.
Charlie - Your "Grips" seem as if they would make a job that I would hesitate at doing in the first place, a lot easier.
This is what makes this forum so great - one problem and so many members willing to help find a solution. Again, many thanks to everyone who helped.
(Bernard Hagan)