Ifitaint,
Apologies to all if I have got it wrong, but while there is that box on the first form, the cross-crediting document states:
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2.1.7.6 CAA-issued licences with a valid SEP rating
The holder of a valid CAA-issued Part-FCL Pilot Licence (Aeroplanes) of UK PPL (A) with valid SEP class rating who wishes to obtain a NPPL (A) (SSEA) shall:
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To me, that means one gets a new licence - "wishes to obtain a National Private Pilots Licence..." - which is different from adding a rating to another type of licence!
It also talks about not be able to "transfer" some rating from one licence to another, again meaning - in my view - that this is not adding an SSEA to a PPL A, but getting a new licence.