I found some references on this page:

http://www.landforms.eu/orkney/Fossils/Palaeospondylus1.htm

but the latest paper they have

Newman, M.J. & den Blaauwen, J.L. 2008, New information on the enigmatic Devonian vertebrate Palaeospondylus gunni. Scottish Journal of Geology, Short Communications, 44, (1), 89-91

(link to researchgate page on the paper: https://www.researchgate.net/publication/213773505_New_information_on_the_enigmatic_Devonian_vertebrate_Palaeospondylus_gunni)

is from 2008.

Article New information on the enigmatic Devonian vertebrate Palaeos...

I have a student who has just completed her project on the animal and concluded that it was a larval lungfish, but perhaps not of Osteolepis. There are certainly quite a number of recent publications out there. There is, of course the article by Keith Thomson in the American Scientist: http://www.americanscientist.org/issues/num2/a-palaeontological-puzzle-solved/3

There is also the latest publication: Johanson, Zerina; Kearsley, Anton; den Blaauwen, Jan; et al. 2012. Ontogenetic Development of an Exceptionally Preserved Devonian Cartilaginous Skeleton. JOURNAL OF EXPERIMENTAL ZOOLOGY PART B-MOLECULAR AND DEVELOPMENTAL EVOLUTION Volume: 318B Issue: 1 Pages: 50-58 that might be useful.

The hypothesis that P. was a larval lungfish has been convincingly rejected by Joss and Johanson (J. Exp. Zool. (Mol. Dev. Evol.) 308B: 163-171 [2007]). However, I am not aware of any new suggestions regarding the affinities of P.