The flower is Alyssum, an exotic, I think the scientific name is Lobularia maritima. I’m still working my way through the various pollinators on it over the last couple of days - three native bees, three or more flies including one bristle fly I haven’t seen before. This compares to almost nothing elsewhere in the garden.
I was wondering what the flower was! I just could not think of any such native in bloom at the moment. My Christine grows Alyssum in summer so I must take a more focussed look on them! You will notice that they have quite a pungent scent which is possibly why they are attractive to flies. As Peter says its very early for bees to be about and as my colleague Stephen Marshall says "Flies rule where its cool"