With Mark's help I identified the pump as CP3 with the optional suction/pressure supply pump in tandem on the rear. The in-tank pump is to supplement the suction head, this being a commercial application with a greater height than normal between tank and pump. Measuring vacuum at the pump inlet gave no reading. Undoing the four torx screws to remove the supply pump showed the drive spigot sheared, as Mark had predicted ,
thanks Mark,
Peter