Thanks Mike - Ive gone through those solutions in that thread, and they all
seem to have their pros and cons.

For example - No you cant deform a surface with a planar cap or it just disappears! But you can easily animated the underlying curve (with cluster deformer/whatever, and the planar cap travels with the end whatever you do quite happily. But its always sharp!

Whereas your technique of adding isoparms and collapsing the end closed creates a surface that can easily be deformed, but if you animate the underlying curve, it goes all wonky.

If only there was a way to get an end cap that would lock to the end of the extrusion, even when animating the underlying curve, and also be able to procedurally sharpen / soften the corner.

Is this possible?