Thetford C402 bench toilet - pump location

Hi All, Has anyone any experience of the C402 bench toilet? The pump on mine had stopped working following winter storage. Using some info I found online I was able to find the pump remove it, and free it up. Works fine again for the moment so all good so far.

However I'm trying to work out where should I place the pump back inside the tank.  Removing the pump was quite a fiddle through the small access hatch and I will admit that I wasn't paying enough attention to the exact position of it before I pulled it out of the tank.  I see that there's  vertical black tube in the front corner of the tank about the same shape and size as the pump. Does it go in there? I've tried unsuccessfully to get it in there, leading me to believe that's not he right place.  Or do I just push it down low in the water?  That's what I've done for the moment and it works OK but it won't pump the tank completely empty from this position.  Thetford's own repair guide and exploded part diagram are vague. I did email the question to Thetford but got disinterested response. I'd appreciate any thoughts on the topic. Thanks so much!