How to deal with persistent weeds in three-month-old turf (SE England)

Please bear with me as I am quite eager and handy but a little less than experienced when it comes to the finer technical details!
I laid nice Rolawn Minster turf about three months ago. I live in Greater London. The lawn has been progressing nicely, although lately I have noticed that a persistent weed has started to spread throughout the grass and I don't know how to eradicate it.
I have tried to attach a picture to this post - this weed is slightly lighter in colour than the actual grass and the blades are thicker and coarser. It grows quickly and has seemed to spread throughout the lawn quite quickly.
I have found lawn-care products that will kill weeds and leave the rest of the grass alone, although most of these say to wait at least 18 months after the turf has been laid - I am afraid I won't have much grass left by then!
How can I get rid of these weeds, other than by ripping them up by hand and laying seed into the holes? (This is in itself unsightly, but it's the best solution I can think of.)
Many thanks
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ajax said:

Since your weed is a type of grass (a coarse, ugly grass), these lawn weed killers probably wouldn't do you much good even if you could use them right now.

You could pull these by hand, or you could *carefully* paint them with glyphosate. Don't get it on the lawn grass or any other plants you want to keep. Glyphosate is a broad-spectrum herbicide.
Either way, it will be a lot of work.

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Agree with the above. It looks like an undesirable grass in which case the broadleaf weedkillers for lawns available at the garden shop are very unlikely to do anything, because, well it's a grass. But I would try some on a test spot because the alternatives are not good. You can always use the weedkiller for regular weeds.
There are some herbicides that will kill specific undesirable grasses, but many of them are only sold to pros that are licensed and they typically come in quantities and price tags that are suited to a golf course, not your yard. And even with these they only work on certain grasses, not all and may not be totally effective.
Best thing is probably to spray it with roundup (glyphosate) using a round cardboard tube so you get as little as possible on the good grass, which it will also kill. You could do it in late Aug, early Sept so you can re-seed at the optimal time. But there is a catch. The undesirable grasses have a lifecycle. Hopefully it's still growing and visible then. Also, it's developing seed heads right now, which isn't good. If you do wait to kill it, keep it mowed on the short side, so less seed heads will survive.
The lifecycle concept is the final treatment angle. If you can get someone to identify it for you, then you can look it up and learn what it is. You can see some seed heads forming. At some point, those will want to germinate. Could be they germinate in fall or spring, etc. If you know when, you can put down one of the pre-emergent crabgrass products at the right time, prior to it germinating. Designed for crabgrass, they will also prevent other grasses and weeds from germinating as well. It's a common product here in the states, don't know if you folks even have crabgrass over there. I could send some.....
One thing that's important is the sooner you get it out of there the better. The more it spreads, the harder it gets.....
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