The shelling, at least, seems to be well-attested, as Northcliffe's gardener's wife was killed in the attack and some of the shell damage can still be seen at Elmwood, but none of the sources I've seen mention it provide any useful information about the ship that would have been involved.

Who was in command? Did the crew know the full details of their mission? And do we know that the targeting of Elmwood really was deliberate? That such a focused attack could be made in the first place would not surprise me, given the extraordinary degree of contempt in which Northcliffe was held by the German high command, but it's still too provocative a story to be simply taken for granted.