You don't have to replace the whole seat - just the hinges.
There is a grub screw at the rear. (Use a hand mirror to see it if necessary). Get the right allen key and loosen from both hinges and take the seat/cover assembly off the toilet. There is a chrome spigot on each hinge. Remove them. If you ruin them, you get new ones with the new set of hinges anyway. Four screws are revealed now. One pair removes the seat cover, the other the seat. Buy a set of new hinges on the internet and replace. You maybe can reassemble the seat only whilst waiting for the hinges.
Gordon Berlyne, November 2011