This is a real newbie question here. I have surface created using surves and the birail tool. I have then projected a curve on it and trimmed part away. Now all I'd like to do is give it some depth. I can see I can offset the surface, creating a second surface nearby with a gap between - but is there any way to do this and automatically generate "sides", like the bevel function does with normal curves? Apologies for any incorrect terminology. Perhaps my approach to making this model is just wrong.
No, unfortunately the offset doesn't build sides. You can build them yourself and have them follow the construction history of the top and bottom surfaces though.
1. Right click on the top surface and choose Isoparm
2. Select the edge of the surface, it should turn yellow
3. Right click ont he bottom surface and choose Isoparm (the first yellow line should stay selected)
4. Shift+Select the same side edge on the bottom surface.
5. Perform a loft.
6. Continue for the other sides of the surfaces until you create a closed shape.