There is only one question in your first post:
However, is there the requirement to sit the other PPL written exams?
The answer is no.
As far as I was aware, the conversion from the NPPL(A) with SSEA to LAPL(A) was just a case of filling out paperwork?
Yes, that's what I said:
An NPPL(A) with SSEA -> LAPL(A) conversion is considered a like-for-like swap so no additional exam requirement.