On a whim last night at 10.30 I sent the picture to the Met Office. By 5.30 this morning I had this reply.
The formation is caused by a delicate process. The cloud is a thin sheet of Altocumulus at an estimated height of 18000 ft, and it was most likely, originally, one solid sheet of supercooled cloud droplets. Supercooled droplets are well below freezing (i.e. -10C to -18C) but remain in liquid form, unfrozen, usually because the air is so clean and ice has nothing to freeze onto (ie. like frost on a clean window selectively forms on scratches or dirt).
If a disturbance occurs within the cloud, such as a plane going through it (adding microscopic particles into the air), this can kick-start the process of freezing the particles. As they freeze, they form into small ice crystals or tiny snow-flakes, and then fall out of the atmosphere. The cloud is then no longer there. It then becomes a progressive process, as one ice crystal forms, it absorbs the moisture of the surrounding supercooled droptlets and dries the air out. The result is the cloud will slowly dissapear from the point at which the freezing process first started.
It can also be caused by descending air from aloft, drying out the cloud, and causing the droplets to evaporate.
This cloud formation is more popularly known as a "Hole-Punch cloud", which are widely photographed and on the internet.