Hmm. Schedule 8 says:
Airline Transport Pilot Licence (Aeroplane)
Privileges and conditions:
1 (1) Subject to any conditions specified in respect of the licence, the privileges of the holder of an Airline Transport Pilot Licence (Aeroplane) are to:
(a) exercise all the privileges of the holder of a JAR–FCL Private Pilot Licence
(Aeroplane), a JAR–FCL Commercial Pilot Licence (Aeroplane) and an instrument rating (aeroplane); and
I read that as having an IR(A) - which therefore supercedes the IMC rating.