well now,
I did two things wrong.
(I was using an incubator. Mine is a fully manual one, with a probe thermometer, and also an electronic thermo-hydrometer)
I stupidly set the incubator at 35.7 degrees instead of 37.5 degrees and only discovered it on about day 12.
I remember reading that if the temp is too low the chicks take longer to develop so i calculated that they would probably hatch on day 22/23/24 !! and so carried on turning the eggs to day 20 instead of the normal day 18. The chick that hatched did so that night; but the others that had develped must have drowned because they had been turned after they would have broken into the air sack in preparation for hatching.
the weired thing is that the chick that hatched and survived actually hatched early!! so my calculations etc were all wrong. There were eight fully formed chicks out of a batch of 18. So I had drowned seven of them

some of the other eggs were fertile and had developed to about half way! so the fertility of the eggs was not an issue really, concidering the time of year.
The thing is that we cant imitate or replace nature and always get good results. I do sometimes use an Isa brown who oes broody really easily and is a fabulous mum. She always gets a better hatch rate than me

Well the best lessons are learned from mistakes.

emma T