It could be easier to replace the whole deal, I'm no expert and have no idea what type you have. In our modern world replacement parts are harder to find and more expensive than just buying a new unit.
That is Ferguson's logo.They are not a manufacturer,rather a distributor.Check your phone book see if they have a branch in your town.I have never bought a toilet there,was unaware they sold toilets with their badge.Wonder who made it for them.