In my experience, Pip, they need cool temperatures to sprout. You may be starting them at the wrong time of year. In your area, I would start them around April, and they should start blooming around the end of July. The dates are October and January, here. An exceptionally cool winter may delay bloom for a month.

The self sown ones in the garden wont sprout until March etc. They dont need any special treatment in my experience.

how long ago did you plant them Pip I find that they are not quick to germinate and did you sow direct or in trays(made me look twice-- I wouldn't sow outside until march/april here :0)

I sowed direct, several weeks ago, and when none of them came up I gave up and put snapdragons in there instead. I've been planting seeds in that same area for ages and have never had any problems.

Wait until the ground warms a little and just scatter them...Thats what i do...never fails....:>)

I hope all you folks from Europe are remembering that Pip is from "Down Under", so her seasons are reversed from yours. She also should have milder winters and warmer summers than most of the UK--think southern France, with a little more humidity in the summer. That's if I'm remembering Sydney's climate correctly, Pip! : )
In mild winter climates, Nigella is a winter annual, needing soil temps of around 15º-17º C to sprout. Her soil temps are probably closer to 28º, now!

Thanks Moti and Tugbrethil... I'll try planting in autumn and see what happens.

do you get frost Pip-- just wondered-- I've some sweet williams which are biennial and in a sheltered area in small troughs that survived -12!
you could always sow some in autumn and some early spring before the weather is too warm-- cover all bases --
thereagain it could be duff seed....

We hardly ever get frost, Pam, and never on the plants - just on the lawn.

I'd plant in autumn then or just as it starts to get warm in spring hope they come through for you :o)

Thanks for your advice :o)