Thank you all for the information and pictures.
The name has been repainted (same name and style) several times over the nearly 30 years I have known the boat and I am sure any early evidence will have been destroyed. Similarly, the sides and top have been replaced but in the same style as the original.
Looking at the photos of Larch etc. it is clear that Emma Jane is a close relative. The positions and style of the cooling air intakes and exhaust are identical. The hand rails are identical and the visibility of the internal planking through the external plywood exterior I remember from when we first owned her.
The possibility that she was not built for hire is I think discounted by the original presence of a brass plaque with "Willow Wren Hire Cruisers" on it.
No evidence of bridge bars incidentally.
I think Barney always used Saab engines - this has an original Lister SR2.
I would still like to be sure who built her and when.
Thanks again everyone.