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 -
http://i242.photobucket.com/albums/ff272/oliverclotheshoffe/misc/ztransoff.jpg

With Ztransp on -
http://i242.photobucket.com/albums/ff272/oliverclotheshoffe/misc/ztranson.jpg

Any ideas ? Thanks.