Bought a Seat Ibiza in November 2009. Just noticed crack in windscreen. It is not a 'chip' like from a stone or other object. It is a fissure, a line crack. Could this have been caused by the extremes of temperature during the recent cold snap or could it be a fault or stress fracture actually in the glass?

It's tempered glass so it can handle the extreme to a point unless you live in the south pole. A crack will always happen if not immediately when a solid object hits the tempered glass. Characteristic of tempered glass is that once it's gone through it's tempering process cannot be cut or gouged for fitment adjustment because it will always crack rendering useless. Unlike untempered glass you can gouge and cut it as many times to fit it in place.

What it means is when ever a tempered glass gets dinged hard enough to gouge it, it will most certainly crack. But for a crack to happen without a gouge because of temperature changes are unlikely because of the tempering process will withstand this changes except subjecting it to a fire above it's flash point.