The postcode question is more complicated than I thought.
A 6 digit postcode can entered, in a slightly convoluted way:
where to>addresses>search area>enter city/postal code>enter post code.
The post code is found, but you still need to enter a house number - any number seems to work. Trying to enter street fails. Also, if the GB Topo Pro maps are installed, even if they are not enabled, only a 4 digit search works.