Dual Flush Push Button Toilet Leaks into Bowl Even After Replacing Flush Unit?

[Rincewind]

I have a dual flush push button toilet. Its started leaking into the bowl. I have removed, cleaned and replaced the flush unit but it makes no difference. It seems as if the central pillar (with the rubber disc on the end that seals the hole into the bowl) does not extend freely far enough and I cant see where it is sticking.
This picture

shows the central pillar fully extended but it only extends under gravity about 1/4 of this distance and I have to pull it to get it to extend the rest of the way.(If I install it like this, there is no leak. Once I press to flush that pulls the disc up and it does not fall back down again so water runs into the bowl)

So really I have two questions:
Any one know how I can take this assembly apart to see what is sticking or indeed know what can be done about this sticking?

Can anyone identify the make/model of flush unit so I can, if all else fails, go buy a spare part.. The full system is shown here

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Thanks in advance for any suggestions

[Rincewind]

Well that wasn't too hard. It seems the flush valve comes apart in three sections held together with two plastic clips at each join. A bit of levering with a screwdriver (and a lot of crossed fingers that nothing broke!)and they pop apart giving access to the internal mechanism.

It seems the central column (with the rubber seal on the bottom that was not extending properly) had come off its guide rails. Once it was properly seated it travelled up and down the full distance under its own weight (as opposed to getting stuck 1/4 way down)

Reassembly was pretty straight forward - the three sections are just a push fit and the clips engage and it now all works just fine

Hope this is of use to someone else