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From LASORS:
JAR-FCL Professional Aeroplane Licence Holders
JAR-FCL CPL(A) and ATPL(A) licences does not have
in-built IMC privileges. The holder of a JAR-FCL
professional aeroplane licence without a valid IR(A)
who wishes to obtain an IMC Rating must complete the
following:
i. 5 hours of applied dual instrument instruction in
accordance with the IMC Rating syllabus and;
ii. pass an initial IMC Rating Flight Test.
The holder of a UK issued JAR-FCL professional
aeroplane licence with a valid single-pilot IR(A) does
not require a separate IMC Rating endorsement as the
privileges are included within the IR(A) privileges. If,
however, you wish to obtain an IMC Rating you will
need to apply to PLD for endorsement. The normal IMC
rating issue fee will apply unless application is made in
conjunction with an Instrument Rating (IR). In this case,
the IMC Rating will be valid for a period of 25 months
from the date of the IR(A) skill test and will not need to
be revalidated if the IR(A) remains valid. If the IR(A)
lapses an IMC revalidation flight test is required.
The holder of a JAR-FCL professional aeroplane
licence with a valid multi-pilot IR(A) is not automatically
entitled to fly using the privileges of an IMC Rating in
single-pilot aeroplanes. They will be required to pass a
IMC Flight Test and apply to PLD for a separate IMC
Rating endorsement.