Washing Machine Drain Hose: One-Way Valve Preventing Drain

[Ultuous]

Just moved into a new house and found that the washing machine (Bosch in case it makes a difference) errored whilst trying to drain on a test run... I know next to nothing about plumbing, but on closer investigation there appears to be plastic valve inside the drain hose where it connects to the sink trap: pushing this down allowed the machine to drain and complete the wash cycle (after holding it over the sink for the best part of an hour ).

Having looked at the installation guide (and assuming they fitted the supplied hose - no sign of it anywhere else!) there's no mention of this valve; the only thing I can find that isn't 'to process' is that the they've not fitted the clamp (I found this loose in the kitchen draw)

Can anyone advise? Is this valve a flood protection device that would be 'released' by fitting the clamp (I noticed that squeezing the hose by hand did allow some flow without pushing the valve in) or have they used the wrong hose/ trap connection/ fitted it the wrong way round (i.e. is it there to prevent sink water from flowing into the machine?... as it's built in I can't investigate this theory without pulling the kitchen apart!)

The obvious thing to do would of course be to get the contractors back round to sort it, but I'd rather not take time off to have them back on site if it's a simple case of needing to connect the clamp or similar!

[Ultuous]

Thanks guys - rubber end does indeed slide onto spiggot, but there is what looks very much like a bung (the black object in the second image) stuck inside it!

[Ultuous]

Thanks - it seemed pretty tightly jammed in but didn't see it as a 'foreign object' at the time so didn't try in anger to remove it... back in my old place now, will see if I can squeeze it out tomorrow night!