The reason you are doing an IMC rating or night qualification is because you are NOT qualified therefore it must be instruction i.e. DUAL
SPIC is only used on Integrated courses of training for a CPL/IR or ATPL and has a specific purpose on that course. It can only be flown IFR.
Guidance is available in LASORS Section A App B here:
http://www.caa.co.uk/docs/175/SECTION%20A.pdf Alternatively JAR-FCL1 SubPart A
http://www.jaa.nl/
There are cases not covered in thse documents neither of which are Law. If in doubt read the ANO Article 35 and make you own mind up what to do. It is after all your own personal log and the only legal requirements are in Art 35.
If you are undergoing differences "training" it is really Dual, if you are being checked out i.e. to all intents and purposes you are acting as PIC then there is no reason why it should not be logged as PIS (PICUS) provided it is countersigned by the PIC. No, this case is not in LASORS or JAR-FCL but it does comply with the requirements of Art 35 in that it fairly describes your operating capacity, and the Instructor qualifies as PIC. I have done it this way for years without any problems.