Arachnophobe's nightmare!

Not sure what is going on here but it is certainly something I have never seen before - dozens of little spiders (?hatchlings) scrambling very actively about on our external kitchen window-sill this morning - scared the wits out of Fiona, who detests spiders.
Up close these little guys are actually quite pretty - if you like that sort if thing.
Any suggestions as to what is going on?