The walls of our 1930's house appear to be thinner upstairs than
downstairs, most obvious on the landing where there's a shelf / second
window sill:

http://tinyurl.com/3a8tuo

Our neighbour told me that, when she had cavity wall insulation
recently, the installers were only able to do downstairs. She wasn't
sure whether this was because upstairs was solid or a thinner cavity.

Has anyone come across this type of construction before? Is it more
likely that it's solid or just a thinner cavity? I'd have thought that,
if a cavity existed, it could be filled with the kind of blown
insulation that's used today.

Thanks.