HOughy, my first thought was a circumzenthal arc:
http://scienceworld.wolfram.com/astr...nithalArc.html
but this requires the Sun to be fairly low, an unlikely event at midday in midsummer.
It is possibly a fallstreak hole
http://xenmate.blogspot.com/2005/12/...ch-clouds.html
where ice crystals are forming, and you just happen to be at the right angle to get a prism effect.
most unusual