If you convert to a JAA CPL-SE and then you do your 15hr conversion on a multi-IR, you will also have to complete a multi-engine rating flight test. If you do it at Aerodynamics Malaga, you will do a six hour course on a multiengine plane plus a couple hours on a sim followed by the flight test which is pretty basic. This is in addition to the Multi-IR conversion course and assuming that you don't have a FAA Multi-CPL. It is not a full CPL checkride like you would have to do for the FAA. In the UK, it seems more common that you will do the Multi-IR checkride followed by the Multi rating skills test. But to answer your question, you WILL have to do a Multi rating skills test, because a Multi-IR skills test does not give you the multi engine rating, only the Multi-IR rating.
If you do your initial CPL skills test in a multiengine, you will not have single engine privileges unless you also do a single engine rating skills test (this is assuming that you don't have a JAA PPL)