Thanks for all your replies so far. Perhaps I didn't make it clear enough
in the original posting that the smell I was referring to was the one you get
from concrete or tarry surfaces, and not the more rural damp freshness, which
has something to do with rotting vegetation, I think.
Any further ideas welcome before I email my niece late tonight with a
distillation of this wisdom.

Tudor Hughes, Warlingham, Surrey.