Twice annual weed and feed does not touch the yarrow in my lawn.
At present the lawn is totally brown, and the green yarrow very evident and easily identified. It seems the ideal time for treatment.

Can you recommend a treatment to eliminate it, please?

Yarrow, aka wild carrot, comes back year after year from a carrot-like tuber. Your best bet may be to dig them out.

Bill, yarrow and wild carrot (aka Queen Anne's Lace) are not the same plant, but certainly both can be weeds.

Thanks Bill, but the problem with digging the yarrow out, is the sheer quantity. The lawn takes me 1.5 hours to cut (Hayter 48), and I would estimate there to be probably 30 square yards of yarrow spread over 10 areas. Yelmottob.

There are some UK-based suggestions for yarrow control in this RHS page:

https://www.rhs.org.uk/advice/...

I thought that Yarrows common name was wild carrot. This old brain, Bang Bang, thud thud. My bad.
That's for the correction.
I do not know what to tell you. You sprayed it and that didn't work. You don't want to dig it out. There is nothing left.
And the bad part is that unless you remove it completely it can come back from small pieces of the tuber left behind. Another option is to use a "back pack sprayer" and walk the lawn spraying an herbicide on each individual yarrow. And you'll have to repeat that process probably for several years. An invasive weed has entered the area and has taken a strong hold. There is no magic bullet that I can see. Good luck!