Generally, matching numbers and a good restoration add 100% to the value of a Black Shadow. The D Shadows, although they are a much better riding bike than the C Shadows, do not have the "looks" that buyers want and they sell for a bit less than C Shadows.
A Black Shadow with a non-original Vincent engine would very likely not get the 100% premium that "collector" bikes would receive. At 40,000 Pounds Sterling you are into the territory where a matching numbers Rapide is starting to look viable as a better collector option.
There is a big risk factor regarding the bike's inability to appreciate with other Vincents because there are so few comparables. If this were a private sale and a friend was asking me about the value, I would say start at 25,000 Pounds and go up very little and carefully. As a collector bike, it is not worth fighting for.
David