Does anyone know what size the coolant drain plug bolts are on the front two cylinders?

I've tried a 3/8 and 9mm which are both to small, yet a 10mm is to big...?

I haven't tried, but I would of thought it was a 10mm head on them. Could be imp bsp but I very much bought it, maybe just a worn down bolt head. have you tried a 6 point socket rather than the usual 12 point sockets?