The simple Z shaped piece (see photo) I've tried to do by resin infusion, but both the internal and exterior bends are resin starved.

mould is simple 3 bits of plastic, so nothing complex.

Would I be better doing this be wet layup and vacuum afterwards, rather than infusion ?

I would say that this has bridged and didn't have enough resin to fill those voids. You need to make sure the fabric is really into those corners.