if it happens again a temporary fix is to fix a plastic bag over the open vent with an elastic band,it would blow off if there was a build up of pressure but should stop pumping over. :
shutting down the cylinder is not a proper fix The positon of the cold feed and open vent in relation to the pump is the cause ,you may now find that your cylinder is slow to heat up.
Tte open vent and cold feed should be connected before the pump ,no more than 150mm apart ,the sludge in the system would suggest that pumping over has been happening for some time.A quick fix to get you out of trouble would be to fix a plastic bag over the end of the cold feed with an elastic...