Hi Lewis, I've attached the result of using the untrim/retrim technique as I described above. The surfaces that you get after untrimming are perfectly usable, they just extend over top of each other so it's easier to manage them by only untrimming and retrimming one at a time, that's all.

This version should have the 100% exact same surface shape as your original file since it actually uses the same surfaces.

If you want help with making a new surface using Sweep please post the curves you are using, it looks like they need to be edited to be smooth instead of having a kink in the middle of the curve.

- Michael