Well, £0 tbh because if the relatively experienced owners who’ve had all potential issues investigated haven’t solved the problems, maybe it cannot be fixed.
If you said it napped/reared at competitions but was rock solid hacking then the horse has a worth.
Are they going to disclose the issues to a buyer? They’ll probably get £6k for him if they don’t, but with that history I don’t think there’s going to be a queue.