Hi Alan/ portbee,
Welcome to the forum.
Try the beautiful solitary bee Andrena cineraria the Ashy Mining Bee [see Steve Falk’s bee book, Plate 4, Page 125]. Several years ago, I once saw a large aggregation of these bees in Wakerley Woods, Northants.
Urbanbumble has already identified this bee for you.
Please visit the BWARS (Bees, Wasps and Ants Recording Society) website. They are collecting observations of 4 common spring bees. Follow Home>Submit records>Spring bee recording>Andrena cineraria. You can submit your observation and photos. I am certain they they will be interested to receive your observations to add to their records and distribution map.
I have recently seen a mass nesting aggregation on a dry sunny bank covered with short grass at Upton Grey in NE Hampshire.
Thanks both for your information.