Aitch, if you do have an authentic Will & Fink, the first of all DO NOTHING to try to clean it up unless you happen to be trained in the art of museum quality restoration. ANY amateur cleaning will devalue the knife. There is no such thing as good condition for its age. It's either in good condition or it's not. What you think is good condition "for its age" and what I think could be worlds apart. If we forget about age as a factor of condition then we might agree it's very good, good, average, poor etc.
Adding a sheath which is new, even period correct will not significantly increase the value, if at all. The only value adding a sheath will be if it makes YOU happy. If you can come up with a sheath that is the same age as the knife, then you might add value.
Considering the possibility that I don't know what I'm talking about, why don't you post a picture or several pictures in Bernard Levine's forum here and let the real experts have a go? In the meantime, just enjoy owning the knife…..don't try to improve it just yet.
Paul