OK, let's try to clarify the position with modular courses for the IR added to a UK licence or JAA licence issued in the UK.
The length of the course, for a multi-engine IR is 55 hours, of which up to 25 hours may be instrument ground time in a FNPT I, or up to 40 hours in a flight simulator or FNPT II, if agreed by the Authority. (Remember that, under JAR-FCL, the term 'flight simulator' means a full flight simulator in accordance with JAR-STD 1)
If you have a UK PPL with no IMC rating, you complete the whole course.
If you have a UK CPL or UK PPL with IMC rating you get a 12 hour reduction and, therefore, need to complete only 43 hours of training.
If you have a JAA PPL, with or without IMC rating, you complete the full 55 hour course.
If you have an ICAO (including JAA) CPL, you get a 5 hour reduction and, therefore, need to complete only 50 hours of training.
Possession of a non-JAA IR gets you no credit whatsoever. (But possession of a non-JAA, but ICAO, CPL gets you 5 hours)
You cannot get both the 12 hour and the 5 hour reduction (i.e. a JAA CPL with IMC rating gets only the 5 hour credit)
I think that covers all the options.........Doesn't it?