Head holding tank exterior outlet switch

[benibell]

Hi all, second post. I have another small conundrum. The head when its pumped seems to expel the detritus into the water through the exterior vent instead of into the holding tank. I think this means a blockage between the head pipes and the tank but am unsure. The last thing I want to do is take this pipework apart if I don't have to so any help here would be great.