Was the grass yellow / yellowish before you applied the lawn feed?

Oh Dear! I "Aftercutted" my lawn yesterday! I do hope it doesn't have the same effect, Sunbeam!

No, I don't think so, some areas look absolutely fine, but where it's yellow it also hasn't grown much at all - whereas the green areas are very lush and getting very long again.

It might just want aerating or scarifying in these spots.

Perhaps the roots are being somewhat starved due to compacted soil and the Aftercut feeding ingredients are not getting to where they are needed most.

Try pushing the tines of your garden fork into the soil, then after the next mowing apply some more Aftercut.

Perhaps those more yellowy patches were the bits you might have missed with your 'Aftercut'? I did my lawn a while back with moss killer/feed/weed killer combo. There was a huge navy blue cloud looming. I whizzed round and, of course missed some bits. The lawn looked a disgrace for a while, in spite of the deluge that followed the application. Now, many deluges later, it is looking acceptable. Try some liquid fertiliser on the yellowy bits?

Thank you for the good ideas - it's so good to know there's somwhere to raise all my questions and that someone will try to help. I bet I did miss bits with the Aftercut so will aerate it as suggested and then after the next cut (if it ever dries up!) I'll use either liquid feriliser or the Aftercut again.