The requirements change slightly, depending on which country you are in, but the result of EASA will supposedly be that conversions will be the same in each country.
For example, the JAA requirement for converting an instrument rating is training required determined by the authority. In the UK, it is 15 hours of instrument training in Europe. Under EASA, the UK will adopt the "training as required determined by the authority" or whatever verbiage they use.
I recommend you check out Lasors
LASORS 2010 | Pilots | Personal Licences and Training
But basically, the requirements to convert in the UK if you only have a CPL are to pass an initial class 1 medical exam, enroll in an ATPL theory course (distance learning or in-residence) and pass the 14 exams, pass a CPL flight test, receive the minimum training required for the IR and pass an IR flight test, pass the radio telephony exam, and that is all I can think of at the moment.