M.85,
As far as I remember, in order to fly single crew, all you need is a proficiency check as such, that is, do a proficiency check (PC) on any single crew aircraft, doing checklists, radio and all yourself, and you should be good to go. This goes for all, if your latest PC is done in a multi crew environment (as in sim with a copilot, instructor in the back) or with the intructor as NFP, then you cannot fly in single pilot operations (unless your former single pilot ops PC is valid).
When you get a job, you would have to do an OPC anyway, in this case, it does not matter what your last PC/OPC was regarding single/multi crew.
Hope it clarified it, otherwise, look it up i JAR FCL ...