Toilet flush button / heigh adjustment

The toilet cistern in my bathroom has a flush mechanism very similar to this one:-
http://www.lunns.net/cisternspares/siamp_cf2498cp_advent_sf.jpg
There's a chrome button and a button surround which sits on top of the cistern lid - depressing the button activates the flush etc.
The problem is that the button and its surround no longer fits securely in the lid - I suspect the reason for this is that the height of the mechanism inside the cistern has dropped down slightly.
Looking at it, there appears to be a height-adjustment mechanism for the flush button with what looks like 2 ratchet-like bars either side of the valve assembly. The notches correspond roughly to the height amount it would need to move upwards so that the button would be properly secure again, thus leading me to think that it can be adjusted.
The question though, is how do I move it up one notch - what do I have to do ?
Thanks.
--
Neil Barker

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