What's this in my lawn?

In early September I re-seeded my back lawn. I first killed off everything with Roundup then scarified and removed all the debris. Shortly after preparing the soil and sowing the grass seed I noticed the whole area was being covered by these tiny "plants"(before the seed had germinated). Obviously I couldn't do anything about it at the time but recently I've tried killing the "invader" with a spot weeder, a granular weed & feed and Weedol lawn weedkiller but nothing has touched it.

If anyone knows what it is and how I can get rid of it, I'd be grateful to hear from them.

 

Once the lawn is established next year maybe, if it's still a problem after regular mowing, weed and feed might sort it out.
Not sure what it is.

 

Looks like a lonely myosotis covered by creeping buttercups. It’s a devil to control once it’s established but can be easily treated by most selective herbicides and it will drain potassium from the soil. Your grass will struggle sadly.
Normally it thrives in wet clay based soil but we have had a very wet period. due to the amount of rain and as it likes freshly disturbed soil as well that’s why you are seeing it.
I’m afraid it might be a case of starting fresh if the lawn is covered with this as the grass npmay we’ll struggle.
Best bet now is to see how it pans out but my guess is while you killed off the turf with roundup you may have spread it via roots that were chopped up when scarifying. Add the wet spell and hey presto it’s loving the conditions.

 

Thanks for your replies. Looking at pictures on the net, I'm sure Liz has correctly identified the creeping buttercup. I hope I can get rid of it but I'm a bit concerned that Weedol lawn killer didn't seem to touch it. Is there a more suitable herbicide available? Presumably there's no point in trying again until the spring.

Liz, if I can't get rid of it and have to start again, what should I do differently to make sure it doesn't happen again?

 

It should be easily controlled to be honest but it needs to be actively growing for a selective herbicide to work well. If you tried to treat it in lower temps the success rate drops.
I would now wait till the spring then see how the grass is and go from there.