Grohe dual flush toilet

Grohe 42357PI0 EAU2 old style dual flush button

Is it possible to service/repair the bellows assembly on the above ?

I'm not sure whether the problem is on the button or the cistern so I want to eliminate the button first. The problem is that it only flushes (on both flushes) when the button is held in. There doesn't appear to be any obvious problem with the button assembly on removal. It is about 5 years old.

Any suggestions welcome.

AC