How can I render light being reflected on a surface but keep the surface invisible?

Question

I have an object which is catching and reflecting light from an emitter.

I want to keep the light but remove the object.

In the attached image I want to keep the main circular object in the middle but remove the background object - however I want to keep the light that is being reflected on it.

I have created render layers to seperate the items and render them individually but I can't figure out how to do this.

Any ideas please?

Thanks