You should check AMC/GM to part-FCL:
AMC1 FCL.720.A(b)(2)(i)
Parts of it copied below for those who are interested...
(a) A number of aeroplanes certificated for SP operation have similar
performances, systems and navigation capabilities to those more usually
associated with MP types of aeroplanes, and regularly operate within the
same airspace. The level of knowledge required to operate safely in this
environment is not part of, or not included to the necessary depth of
knowledge in the training syllabi for the PPL, CPL or IR(A) but these
licence holders may fly as PIC of such aeroplanes. The additional
theoretical knowledge required to operate such aeroplanes safely is
obtained by completion of a course at an ATO.
(d) Demonstration of acquisition of this knowledge is undertaken by passing
an examination set by ATO. A successful pass of this examination results
in the issue of a certificate indicating that the course and examination
have been completed.
(e) The certificate represents a ‘once only’ qualification and satisfies the
requirement for the addition of all future high performance aeroplanes to
the holder’s licence. The certificate is valid indefinitely and is to be
submitted with the application for the first HPA type or class rating.