From what searches i've done on internet you will probably have to replace the seat I cannot find anything refering to the soft close hinge/s which I've no doubt you've have found hence the question.
Nobby
December 2008
I can answer this question...
Where can I purchase a new hinge for my Roca soft drop toilet seat?
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix toilets.