In article <30F27F0C.4330@parallax.co.uk>, jasonp@parallax.co.uk says...

>back again. Okay, here's a common identified factor, which I'd now
>like someone to explain why?!
>All the underexposed shots are portrait format and the well exposed
>shots are landscape format. My flash unit was fitted to the top of
>my camera on the built in hotshoe there.

I suspect that the flash contact (or batteries?) loses contact with
the hot shoe contact when the camera is vertical. Try adding a PC cord,
or tightening down the flash on the shoe (or cleaning battery contacts
and bending out the flash metal battery contacts). If all that doesn't
work, try contacting us again ;-)
Hope This Helps