Osmia caerulescens, Blue Mason Bees yesterday in West Sussex.

 This green-eyed bee was investigating a hole in a fence post in Cuckfield, West Sussex, from where a six inch nail had been removed some years ago.

 Thanks to Kim Leaver on https://www.facebook.com/groups/1590641777855221/?multi_permalinks=2260694824183243&comment_id=2262001780719214&notif_id=1555930558226114&notif_t=feedback_reaction_generic

it is identified as a Blue Mason Bee male, Osmia caerulescens.

And above is the female from the same day in the garden.  Many thanks Kim for your help and advice.