Grass going to seed

Posted before about my new turf problems with bare patches.Seems to have at last filled out quite well. I've been leaving it reasonably long before mowing but large areas seem to be going to seed very early. It's a bit unsightly and I can only describe it as 'stalky' growth plus the flowery seed heads.

I don't suppose anyone has any bright ideas to eradicate this,(short of digging it out?) Many thanks.

It occurs to me that this may be the remnants of the 'old' lawn coming back to life as I only turned some of it over and didn't actually remove all of it. Would that make sense?