Question
Have been trying to grow the above from seed for the past two years, seeds come up a treat follow instructions to the letter, the first year after planting out only a few took the biggest one got to about 10 inches and no flowers,last year had about 3 got quite tall not as tall as they should 2 flowered but not very well help! i live in Ilford Essex clay soil but i have been inproving the soil for the five years i have live here,i dont really have trouble with any other seeds

Answer
Carol,
I am assuming that you have started the seed indoors or in a growing house, yes? Here are some thoughts about this plant -

Typically, N.Sylvestris seed started in late march and transplanted to the garden in late May, will not flower until late July.  Therefore in this area we tend to start the seed in February or early March, and plant out in late May.  If you can put tender annuals out earlier than that, you can start the seeds now.

My Ball Culture Guide notes the following about N. Sylvestris, and it sums up most growers experience:
"N.Sylvestris can be nonuniform to germinate and develop.  In some years seed will germinate relatively quickly while in other years seedlings develop over a period of several weeks and the resulting plants are hard to uniformly size up."   Plants that germainate later will not get as large.  For this reason, we don't see many commercial growers offering this plant on the East coast of the US.

Because this plant gets large, I would also put the seedlings in large pots as soon as they have two sets of real leaves.  If grown in containers that are too small, the crowded roots  can send the message to the plant that large growth is not possible. Some plants recover after being planted in the ground, but some do not.  All the growers I know plant N. sylvestris in 6" pots or larger.

I would also fertilize this plant every other week once it is put in the garden - with a mild liquid feed.

I hope this helps!
all the best,
C.L. Fornari