Ladybird, Ladybird

I've spent a lot of time in the garden over the past few days making the most of the mild weather and the last few days of recreation before I start my new job next week.  The thing that has most struck me is the huge number of ladybirds around the place. 

A real pleasure to see so many, they should help keep down the greenfly population, though I'm wondering what has caused such a population surge?