Peugeot 406 engine wont start

My battery went flat today whilst I was at work. I got a new battery and changed the battery over. Once I started the car it was ok, I travelled down the road, however every time I put my foot on the brake the engine cuts out, leaving me with no power, etc. I managed to get to my destination but it cut out again. I couldn't restart the car at all. I put the immobilizer code in and the electrics came on but when I turned the key to start the car, nothing happens. The engine light comes on which suggests an engine fault but wondering if anyone has any suggestions what could be causing this. The lights on the driver side stay on constantly, which is probably what's draining my battery. Any help would be greatly appreciated.