What on earth were a couple of bobbies (more like boobies from this typical stereotypical representation) prowling around in the middle of the night around Carfax Abbey? Just at the time Dracula is being killed? And why was van Helsing left alone by his fellow vampire-killers?
YelBard1 may be right that the two constables heard Renfield screaming and came in Carfax Abbey. Van Helsing may wanted to be left behind so that he would get rid of Dracula's staked body so that trouble won't be caused.
That does work. The only real inconsistency is that the fact that the sun had come up at the end of Dracula -that was why he'd retreated to his box of earth- but at the start of this film it was night again.