To Chas and anyone else still interested, I had some weed grass growing
in my lawn that looked identical to the photo that Chas had provided a
few weeks ago. I let it grow to seed head and managed to identify it
as likely to be poa trivialis. Took it to the local county agri
service here and the expert there agreed. It's common name is rough
stalk bluegrass.

The bad news is that there is no selective herbicide for control and it
will spread. Roundup and reseeding, pulling, mowing it frequently
enough to limit seed formation, applying pre-emergent crabgrass control
in early Fall are some of the ways to deal with it.