Actually, Dicky, there's very little difference between a rendered wall and a facebrick one. Outer leaves of a cavity wall are designed to take in water. This leaves by evaporation without passing into the inner leaf. Modern CWI will not transmit moisture across it in most parts of the country.
I say "most parts" because the NHBC, amongst others, produce a map of the UK showing regions which are classed under "extreme exposure". Even in these areas, CWI can be used, but other measures are required to lessen the effects of severe exposure.