Whopity, the NPLG site is correct - don't confuse the issue with ponderous ANO references. A Skill Test is not required.
noblues - your NPPL/SLMG pilot completes differences training with a FI on SSEA aircraft and sends the assoicated paperwork plus cheque off to NPLG Ltd. They check it and forward it to the CAA, who add the SSEA Rating and send it back to the pilot.
This is NOT the same process as completing differences training for tailwheel etc conversions, so the FI cannot just 'sign off' the pilot and allow him to fly - he has to wait until his licence includes the SSEA Rating.