The grass may well be a native volunteer (possibly from a bird resting in the tree. I let a few native grasses grow in a mulched area we keep. The birds take the seeds and lots of birds stopped by to pick up the brown leaves for nest material in spring. With so much land mowed, the birds have a tough time finding nesting material.

I also let a patch of moss grow in another area- more nesting material.

From my experience, grass will take over your garden. Get rid of it. Keep a few specimens if you like, but don't let it go to seed.

As for flowers, if you like what you see, keep it. You can always rip it out later. But grasses are really hard to permanently get rid of without laying clear plastic over the whole area to kill them.

Thanks - I think I should get rid of them afterall, as there's just too much of them and other weeds there. So it makes more sense to clean the whole place up. I was also uncertain at first because there is a nice high grass patch in my garden edge (from neighbour's weeds) which I pluck occasionally to feed by guinea pigs. It could be seeds from that too, so I might have to get rid of all grasses anyway. Or...with the large grass clump, if I remove seed stalks when they occur, is it relatively safe to contain? But grasses also spread via roots don't they?

I guess if I want grass, I should allocate an area for that :)