IFR requires 2 crew in a pressurised turboprop under the ANO so required if flying PT under an National AOC. If flying PT under a National AOC in a Chieftain then can be single pilot as the ANO has a para allowing SP with an autopilot for IFR in a twin piston.
However most tasks will be categorised as CAT requiring an EASA AOC these days. This allows SP for IFR and Night if suitable autopilot and some other regulations adhered to.
The bit I have an issue with is that even under EASA aircraft with a MOPSC of greater than 9 must be 2 pilot, but how does the Authority classify MOPSC? I know the EASA definition, and following this if I only put 8 seats in a Dash 8 and got that approved in an OM would that make it SP?!!!!! I always thought that the greater than 5700kg weight limit was the game changer - yet I can't find this in the EASA IRs - just references to MOPSC......
Under an EASA AOC you can't put pax in the RHS unless purely VFR now so that's one less.