thanks Linda ... best if I direct you to Les Cowley's site -> http://www.atoptics.co.uk/halo/subsun.htm a fantastic resource

The subsun is the most common subhorizon halo. It is a direct reflection of the sun from millions of cloud crystals acting together as a giant mirror. Plate crystals are the usual source. Rays can reflect externally off the uppermost horizontal face or internally from the lowermost.

we had just taken off, say 10 minutes, as we would be passing Arran when it appeared... then I grabbed the camera from my rucksack and started shooting... im  used to people giving me weird looks on planes