Thanks John for your prompt response to my plea for an ID. For nigh on 30 years the old slide has lain underexposed and dormant in a cupboard; at Xmas I treated myself to a modern slide-scanner. With it I have been able not only to produce an enlarged image of this airplane overhead, but also to lighten up the image enough to read the registration.
Your recognition of it as an S2R Thrush Commander built in 1972/73 as N5668X puts the icing on the cake for me - I was photographing animals and birds at Hai-Bar which I had less bother identifying, so to be able to name the plane now comes with a tinge of extra satisfaction!
Cheers, Stan.
PS: If you or the website has any use for archiving/using my picture, please feel free to do so gratis.