MUNT - see CAO 82.3
Conditions on Air Operators' Certificates Authorising Regular Public Transport Operations in Other Than High Capacity Aircraft (includes Appendixes 1 to 5)
In appendix 4 there is the (some say obsolete) requirement for the pilot in command to have 500 hrs as PIC (or ICUS) in multi-engine aircraft under IFR rules.
This is in addition to anything required for an ATPL.
Other things listed there include 2000 TT, 50 PIC or ICUS on type.
Note this only applies to RPT other than High capacity (ie AC 36 seats or less)
so FRT CHTR etc do not have this requirement.