Terminology and privileges aside.....
Why? Terminology and privileges are pretty crucial
.....how is the SSEA rating added to the UK PPL?
UK PPL or CAA-issued JAR-FCL PPL(A)?
If you wish to add the SSEA Class Rating, you will need to pass the SSEA General Skill Test. But why bother if your SEP Class Rating is valid and you have a Medical Declaration as you can just continue to fly on a SEP Class Rating but restricted to SSEA Class Rating privileges?
Is it sufficient just to undertake the GST and get it signed off in a log book....
No.
.....or does the pilot have to get an SSEA rating added to the UK JAR PPL?
Do you mean UK PPL or CAA-issued JAR-FCL PPL(A). In either case if you really want a SSEA Class Rating rather than simply flying on a SEP Class Rating restricted to SSEA Class Rating privileges, then you will need to pass the GST, apply to the CAA and pay the fee...
If he was just revalidating an SEP(L) rating by taking the GST then it would be signed up as a renewal and no fee would be payable to the CAA.
You cannot ‘revalidate’ a SEP Class Rating by ‘taking the GST’. Either you revalidate by passing a Licensing Proficiency Check or renew by passing a Licensing Skill Test.
However can the examiner just add an SSEA rating in the same way or does the rating have to be 'issued' by an appropiate authority?
Examiners cannot add Class Ratings to a licence.
If not what proof has to be available to say that the pilot is operating legally?
You will need to make a declaration when you apply for a Class Rating – penalties for false declarations are pretty steep.
Also does the NPPL Association.....
There is no such body
......require a copy of the medical declaration - I can't imagine they would be interested as the pilot is not actually an NPPL licence holder. Or should a copy be sent to the CAA instead?
To add any Rating to a UK or CAA-issued JAR-FCL licence, you must apply to the CAA.