Thank you all for your comments.
WRT vibration in the set up - I would be surprised if this were a significant factor as the photo was taken indoors, the subject and the camera were both well supported with the shutter tripped (naturally) by cable release. As noted, the camera is a TLR. Parallax was taken care of by a 'paramender' which puts the taking lens in the same position as the viewing lens. Generally the lack of sharpness seems constant - that is there does not appear to be a plane of sharpness 'behind' or 'in-front' of the plane I was intending to focus on.
On reflection it seems to be that there may be a diffraction effect, or the lens is not up to the job - it is not a bona fide macro lens.

Methinks that I'll do some tests, if only to find out if the lens has a 'sweet spot'.