Push button toilet won't flush properly

[Dark Eyes]

Hello folks,
Our push button toilet won't flush properly most of the time, if you push the button it makes a half-hearted attempt, but gives up; sometimes you have to push it quickly 3 or 4 times and it eventually flushes; occasionally it just won't do a proper flush and you have to leave it for a bit and try later on.
I have attached some images to show the mechanism.
From a quick test, it seems that the button and air tube are working as expected, as the button mechanism didn't seem to have any leaks, and blowing down the tube didn't highlight in leaks in the tube, it also didn't flush the toilet. The air tube goes into a the bottom of the white cylinder, which contains a rubber bag/membrane that is used to lift the white domed weight, which pulls the blue tube up and releases the water into the bowl. It seems that the weight doesn't get lifted much most of the time, so I'm presuming that there might be a leak in the rubber bag/membrane.
I am hoping that this can be fixed by replacing the whole mechanism that the tube attaches to, if so can you suggest suitable replacements, and could I replace it with a dual flush mechanism?
Also there is no obvious stop valve on the cold water feed, is there another way to isolate it locally, or can I hot-swap the mechanism?

 

[oz-plumber]

If you replace the cistern outlet washer it may fix the problem.
They expand over time and can jam on the edge of the canister.

 

[Dark Eyes]

Following oz-plumber's suggestion, I tried lifting up the cistern flush mechanism with my little finger, and couldn't feel any resistance, so I presume that it isn't the outlet washer. I also removed the weighted lifting mechanism and when pressing the button it makes a wheezy sound, so it does seem like this is the issue.

If I am going to replace the flush mechanism, what do I need to know to get a suitable replacement?
Is it the same size hole that they fit into, do I need a specific measurement?
Does it have to be another pneumatic one, or is cable any better?

 

[apcdave]

go for a wee stamp cable job....easy fix , fairly cheap and decent quality

 

[apcdave]

siamp not stamp lol

 

[Dark Eyes]

Are they all designed to fit a standard outlet, as I've seen some product reviews that seem to indicate they differ; if so what do I need to measure to get a suitable replacement.
My current flush button is front mounted in a hole 40mm diameter and 20mm depth, but this could be enlarged if required, and the pneumatic tube is about 400mm long - would a cable one be a suitable/better replacement?