Hi stuart

Attached a model with the offset done. Also a step by step. 1 choose surface and use separate command, 2 select surface and projected line and run trim command for options press done and done this splits the surface. 3- choose the split surface and use the shell command (under the offset command) choose a thickness.
4- delete the top surface and join the rest into a solid. Then you have what is in the attached file.

regards

Jamie