Steve,

I'm struggling with a poor set of options for an aerial at this house. The
greatest vertical section I can get while still allowing top loading is
8-9m, but the lower 3-4m would be up against brickwork. How much does this
affect radiation compared with, say, 6m vertical clear of buildings (with
the same toploading)?

It's fun trying, even if proximity effect robs you of a few dB.  Give it a
go, assess results and then decide what betterment route to take.

73, Bob ZL2CA