Hi Chris, if you select these 2 curves you have on one side here:


you can then run Construct > Loft to build a surface between them and fill in that area.

But I'm not entirely sure that you've got a good shape on the other parts - this area here has a kind of sudden upwards swoop in it, I'm not sure if you're aware of that:





- Michael

Thanks Michael!

I think it is that swoop that's causing me trouble.

I will try to flatten it out and see if I'm able to join the surfaces.

Best,
Chris