On the 17th August, not long before sunset, part of what seemed like a
large horizontal rainbow appeared beneath the clouds.
Looks just like the top of the arc to me.

http://i53.tinypic.com/abtvf8.jpg

shows the complete circle and, as expected, the bit on the left coincides
with the rainbow arc visible in your photo.

Nothing unusual as far as I can see, just a rainbow late in the day when the
sun was near the horizon.