They are quite tricky, there is a key http://stevenstonconservation.org/LNR/AndrenaKey.pdf but suspect someone with some expertise required to help with how to get into using the key.
This is a time of year when they can be quite obvious.
a bee | Observation | UK and Ireland | iSpot Nature
Hmmmm might be helpful for the 21st April First Edition of 
iSPOTLIGHT​:eight_spoked_asterisk:- Andrena Bees 