I've thrown excess turf down at my yard whilst waiting to get rid of it and it has taken root in very well compacted hardcore (I get tankers and lorries in all the time). Don't need a lot of soil for grass to grow.
Thanks for your replies. No I haven't washed the car on the drive for a long time.
It was a visible stripe all last year but really noticeable after winter. Maybe I need to wait a bit and make sure it receives plenty of watering, Miracle Gro Lawn Feed has also been suggested to help it grow.
How soon was the top soil put down after the concrete haunching? I'm no turf expert but could have made soil too alkali or possibly too much salt in the soil from efflorescence