Dual Flush Running Water Problem?

Hi, I have a dual flush toilet that does not stop leaking when you flush it. What I mean by that is, the tank empties into the toilet but then the mechanism that raises up to release the water in the middle of the system (a big red cylinder type shape in my case) does not go down again to seal the tank when it has emptied.

If I lift the lid and push this red thing down the toilet operates fine - fills to the top of this red thing and stops, ready for next flush.

As you can imagine though, I do not want to lift the lid and push it down forever more.

Can anyone tell me what they think may be wrong and what I need to do about it? Is it something I would be able to fix myself? I am not a plumber but not stupid either - just a single mum that cant really afford to pay a plumber if she can help it.

Any help or advice would be greatly appreciated.