There is no reduction in the IR course if you hold an IMC rating. You will need to do 55 hours minimum (50 if you hold a CPL).
That all said, it is a worthwhile rating to get as part of your hourbuilding as it gives you a bit of a headstart on your IR.
olof, the IMC (Instrument Meterological Conditions) is a UK only rating. It is a cut down IR, allowing the holder to fly in zero vis outside of Class A airspace. The minimums for instrument approachs are raised by 200 feet. It is aimed at PPL holders who want the ability to fly on instruments, but don't want all the hassle of a full blown IR.