How to inspect flapper valve in Gala double flush toilet?

I have the Gala double flush type toilet - you unscrew around the push button to remove the lid. The flapper valve is not falling back on it's seating half the time it is used. But I don't know how to get to the valve - do I just unscrew the hollow tube above the valve? I'm scared of breaking it by doing it the wrong way and being unable to use the toilet until correct flapper valve purchased.

thanks

I don't know but why not get the replacement valve first.

The shop advises there are several different flapper valves so I need to bring my old one in.