Manky,
There is some confusion at the moment as there are swingeing restrictions coming in under the new 133, but they're not legislated yet. They will restrict Charter ops compared to what we have now.
But for current ops there is no legislation that requires 5,700kg helicopters to operate IFR for Charter, especially since 5700kg is no longer a reference to anything much relating to helicopters! 2750kg mandates most things helicopterish.
SW, night ops currently require the pilot to be IR for passenger carrying NVFR with the helicopter required to be NVFR approved and multi engine: non pax night ops can be single engine and the pilot night rated only (not IR). Under 133 this will change for passenger carrying Charter Ops with the helicopter having to be IFR and multi engine.