Push button toilet, very old, problem

[jacobball]

Hi

I have a problem with my toilet. It is not flushing at all now. It is an old, low level plastic cistern. It is a push button, but very old, not a round button but a metal rectangular type which says push on the top of the button.

I am going to replace the flush valve but what type would be required, as most of the ones in screwfix and toolstation seem to have round buttons!

Thanks for your help if possible

 

[Charnwood]

It'll almost certainly be a syphon.
Remove the lid and look.

Possibly an old Dudley Elite cistern.