Genghis the Engineer - although we're waiting for the updated version of CAP 3181 to be released, it was clear from the earlier Licensing and Training documents that the CAA intends that the holder of an NPPL(A) will be able to add additional aeroplane Class Ratings by Differences Training alone. This may be delivered by 'an instructor', although it isn't yet clear whether that includes CRIs.
There was certainly no intention for ab initio training to be conducted by CRIs.