[Andy]

I'd press 'd' to view the model non-subdivided - you might see an obvious problem.

Also, check the normals, it looks like one side is shaded differently to the other - press 'n' to toggle the pink lines on/off.

You may have some vertices close to each other but not exactly aligned - use menu Vertex->snap-togehter-by-distance to move them together before Object->Optimize-vertices to remove duplicates.