Have been playing around a bit more. It seems to be happy enough viewing photos which were taken on an ios device (ie all the photos that have been imported from my 4s display fine).
The problem is with photos taken with my DSLR (which I process and then reduce to make smaller copies to store/display on my phone).
It seems to be completely inconsistent as to whether it recognises the vertical dimensions of the photos or not. So, in an album of photos that have all been processed identically and uploaded together, I can scroll through 10 images where the top and bottoms have been cropped off to make the photo fill the screen, and then suddenly there will be one that displays correctly (with bars at the sides). Then it goes back to cropping them again.
Very odd behaviour, I wonder what is happening and if there is anything I can do to correct it.