Last night my headlights switched themselves on as normal, but after a couple of minutes the OS dipped beam went off and I had a dash warning saying there was a fault with it. I pulled over to check and it was indeed out. I don't know why, but I switched the engine off and back on again and the OS dipped beam was working again, only to go off again after a couple of minutes.

I'm going to swap the NS and OS bulbs around this weekend to check if the problem is the bulb, but what should I see? If the bulb now goes out on the NS, is the bulb at fault? And if the OS bulb goes out, it's the ballast?