A desirable 'black' Ariel 500cc ohv from circa 1929. The bikes of the Val Page era at the Ariel factory were nicknamed 'black' Ariels on account of the colour schemes available which took a leaf out of the Henry Ford book. The bike carries a Birmingham registration number. Really no idea what uniform that is that the two lads on board are wearing!
Bu the way the best resource I have found to check out UK registration numbers is provided by the Chiltern Vehicle Preservation Group and is found here.