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. 

Agreed it looks like a small amount of bridging occurred, the product looks like it's slightly resin lean.