He states "As a matter of best practice we would not recommend insulating any cavity of less than 50mm or where it has been part insulated."
The cavity between the inside of the outer leaf of brickwork and the rigid insulation board in the wall cavity is about 25-30mm, so I will take this as a NO then!
So it appears that in dwellings where the cavity is of sufficient width, retro-filling with CWI could be an option. However, in most cases with new builds, CWI should not be a recommendation.
I think this has probably answered my original question now.