I agree. However, I wonder if they actually 'group' postcodes. My street
is pretty well flat, and yet the DTG predictor definitely 'chooses' the
wrong transmitter; I'm pretty sure that it's considering a lot of
adjacent postcodes too.

That is certainly very likely. However, the point is (which I presume you
agree with) that postcodes have quite enough geographical precision for the
task. The predictors probably just look at the first part of the postcode.

Christian.