Particles rendering behind background objects

I have a torch with an emitter doing a fire simulation. Behind this torch is a plane, the material for this plane contains a texture which is a bitmapped image (a .png file with a transparent background). The image is UV mapped onto the plane. When I render the image the particles appear behind the plane in the background. I narrowed it down to the ‘ZTransp’ setting (enables z buffering of transparent faces).

With ZTransp off -

With Ztransp on -

Any ideas ? Thanks.