It's all in section G3.5 of
LASORS The experience requirements referred to in JAR-FCL 2.280 are those for a new JAA ATPL ie those in section G3.2.
Basically you need to do thirteen exams. Under the transitional arrangements you do all the fixed wing papers (14) excluding Principles of Flight and Performance but including a Helicopter Principles of Flight 'in the style of the JAA exam'.
For exemptions see G3.5
Note: ATPL (H) holders who have a minimum of 1000
hours flying experience as PIC or Co-pilot on
multi-pilot helicopters and hold a valid multi-pilot type
rating for the helicopter to be used for the ATPL (H)
skill test and have at least 500 hours experience as
pilot on that type, will be exempted from the
requirements to complete an approved TRTO course
or undergo approved training prior to undertaking the
theoretical knowledge examinations, and the skill test.
Pilots with less than 500 hours experience as pilot on
the type to be used for the ATPL (H) skill test will be
exempted from the requirements to undergo approved
training prior to undertaking the theoretical knowledge
examinations but will still be required to complete an
approved type rating course. However, the course may
be reduced to take account of previous experience on
the same type upon recommendation by the TRTO to
PLD.
The holder of a current and valid ATPL (H) who does
not qualify for the grant of a JAR-FCL ATPL (H), as
detailed above, can still apply for a JAR-FCL CPL (H)/
IR, details can be found in Section D3.5.