Me: "Have you started knocking any of the walls down yet?"
Builders: "Yup, all finished."
D'oh! Ah well, nothing I can do about it now, so I'll just have to make the best of it. The solicitors are coming out for an inspection next Thursday, so hopefully they won't be too annoyed. The walls in question were definitely stud walls, as implied by the original plans, and I've told the builders not to get rid of the rubble until the solicitors can see it (to see that it's basically chipboard rather than bricks). This does slow things down a bit, since I've told them not to start building any new walls until after the inspection, but that's the way the mop flops sometimes.
Ah well, this is all a learning experience, so I'm sure I'll be glad of this knowledge in the future.