Could somebody tell me what this part is and how to fix it?

[londonplumbingadvice]

The toilet flush button on our toilet has stopped working. We tried to fix it and a piece that the button screws into has come off and fallen down the pipe - the toilet will still flush using the internal mechanism but we can't get the button to work. I believe we may need to replace the filler valve but not sure if this is the case. I am also not sure of what type of valve it is (and how to replace it) Would you be able to advise us please?

Many thanks
[automerge]1603108182[/automerge]
I'd really appreciate some advice if possible please?

Many thanks

 

[Aquarius]

Looks like a Siamp optima 49.

 

[Basher]

Shows how the mechanism can be removed:
View: https://youtu.be/cRp_GcFVayI

so you can rescue the missing bit! (or fit a new one)

 

[EvilDrPorkChop]

You'll be able to buy the centre bit and replace by clicking in and out. It's an easy swap. As others have said its a Siamp.

The filler will just be the diaphragm washer - which you can buy here - Arctic Products Siamp Diaphragm Washer - https://www.NoLinkingToThis/p/arctic-products-siamp-diaphragm-washer/5006j