I'd suggest you call a local gliding club (
www.gliding.co.uk will give you contact details) and ask for names of local glider repair specialists.
They work day in/day out with composite aircraft, and understand what's cosmetic and what's structural. Also, I'm pretty certain (glider pilot but not repairer) that fixing double-skinned composite hulls with a foam core is not a job for the amateur. Many gliders use this form of construction, so you'd likely be speaking to an expert.
My experience with glider repairers is that they are happy to give initial estimates and are usually concerned to keep your costs down.
I forgot to add that I think that double skinning with a foam core means that section is structural. However, I would ask an expert.