WIll I poison my lawn?

Hi. I'm about to take up an old lawn and bring in some topsoil with a view to reseeding the entire area. I recently removed 4 huge conifer trees that had taken over the garden. They've left a raised area of land at the back of the garden (from many years of leaf buildup). What I'd like to know is, would this old conifer leaf waste be suitable to rake into the garden (to level off with new top soil) or would it be too toxic for new grass seeds? I can remove it into a skip but i thought if it had nutrient quality I could spread it onto the area where the new lawn will be. Thanks

It would depend on the type of Conifers you removed as your right by thinking some would do not a bit of good however, on thinking the leaf litter has been left to rot for many years, there would be not much toxins left as this would have been absorbed, leeched away or watered or been well diluted.

I would buy a cheep soil testing kit from B&Q or other garden store, cost maybe £ 2-3 pounds and use again and again, but this testing kit is fool proof believe me, IF I can follow the instructions, a blind man can. The results will show the PH of the soil you have, it will give a guide as to what needs added to the soil by way of nutrients like Iron, lime or other stuff, make sure you test a few areas of the garden as it can be different in several area's.
The leaf tiller well mixed into the soil you want to make into a lawn will help to give the soil some body, allow air into the soil and maybe help the soil retain some moisture, all things needed for any plants growing in soil where the roots will be growing for a lot of years.

I would imagine you will be using some form of machinery to till the soil in prep while making a lawn, for preparing the soil you would be better to spread the leaf litter as you go along, also it will allow you to remove the high area left behind when the tree area as it is never a good idea to have high or low dips in the soil when trying to establish a lawn even IF its not needed to look like a lawn bowling green.
As you till and prepare the soil for seeding, you also need to add a feed that is specially for new lawns, this then needs raked into the top inch of the soil AFTER you have ROLLED AND RAKED THIS Area.
There are ,any books, leaflets and info at garden stores for the prep, feeding and watering of a new lawn and after a lot of hard work, you don't want the whole thing to be a disaster just because you did it wrong at the start. Make sure you have the lawn seeded before the weather gets hot or the seedling grass will NOT make it through the rest of the summer, or prep now and wait till end of summer when the real heat you get has passed.
Hope this helps you out a bit.
Best Regards.
WeeNel.

What you add to the area depends on several things.
You do not want to create layers of soil, but rather to create a uniform blend that does not vary over the area of the garden. That way whatever care it needs (water, fertilizer...) will be uniform.

As WeeNel states fallen leaves decompose over time, and even plant material that might have been toxic when it was fresh has probably broken down over the years.

I would spread the material evenly over the lawn and garden, then rototill it with the existing soil.
If the area is still too low then you could think about whether you need to add more organic matter or more soil.

There is nothing magical about 'topsoil' that is required to grow a lawn.
See what you have (soil test is a good idea) then work with that, adding the right material when you know what that material is.