Broad bean cattypillar

I've just been to have a quick look see at my autumn set broad beans. Right in front of my very eyes is a big, fat, juicy, lime green cattypillar munching away at one of the plants. It had eaten at least two full leaves. Bit strange for this weather isn't it? What has laid eggs that have hatched at this time of year? Have the eggs been dormant since last year. I've never known anything like it. I've also had leaves eaten on BB's I've kept potted up in the GH as spares.

The one I got seemed quite a bit bigger. About 2" long and quite fat and juicy. Really bright lime green. How come it's hatched in this weather? How long can eggs lay dormant?