Thank you both for your interest.
I have recently found a couple of pictures which, though they are not of an HF-27, may b of some help.
This is of a pre-war Farman, but it suggests the style of mount for a radial favored by them....
The other is of some early 'one-off' Farman fitted with an 80hp Canton-Unne, shot from a similar 'off the stern' angle; it is not so clear, but something resembling the bearer pattern in the picture above can be seen. I will have to jump through some hoops toget the image down off a PDF file, so I cannot post it just now.
I do not think it is really proper to refer to the HF-27 as a variant of the HF.20 design, and suspect there was some cribbing off M. Voisin in its design. Everything about the wings and the undercarriage says Voisin, and so does the steel framing of the wings, and even the motor. None of these things bear any relation in appearance to the HF-20
et al. Even the nacelle does not match; it seems more reminiscent of a Maurice Farman effort than of an earlier Henry. Only the tail assembly can be said to be in common with the HF-20 line.
Relations between the Farmans and Voisin were, I gather, pretty poor at the time. They had started out as friends, with the Farmans customers and collaborators in refining the design of the original Voisin, but there came a point where M.Voisin sold a machine the Farmans had ordered to an Englishman who offered a bit more cash on the barrel, and that was a final straw in a serious falling out between the two pioneers.
