Thank you for your help.
They all had white spots of varying sizes when they hatched, 8 of the 11 have clear barring on their wing feathers whereby the grey and white barring is pretty much equal in size, the other 3 are pretty much all black with grey around the end of their wing feathers.
I've no way of knowing whether my cockerel carries 1 or 2 barring genes as he is quite dark for a cuckoo marans, therefore these black chicks could be black or very dark cuckoo females.
I'll just keep them all a bit longer to see how they develop.