My query regards an old Howsons toilet cistern, which is perfect except that the flimsy 4.75” diameter transparent diaphragm, which sits above the open spoke piston in the siphon, has ruptured, so the lift action no longer operates the flush.
If no one replies with a spares location then try the following many do rather than source buying the spare diaphragm.
get some soft plastic similar texture ie heavy duty plastic dust sheet type and cut out a new one as it should be accessible to renew the this diaphragm
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix toilets.