Mmmm ...
Pete as per ERSA above .... there as far as I can remember NOTHING that mandates that an a/c above 5700Kg MTOW MUST fly IFR (in daytime) ...
For 'Charter' though the a/c Must be nav aid equipped so as to be IFR capable and at night the pilot MUST be either Ngt Vfr or IFR rated.
My quick perusal of Jepps ATC General Flight Procedures section AU-506 refers ....
Cheers ....
P S John Eacott might be more up to date that I !!!!!