what top dressing and how much should i use on my lawn

is it the clay that is the problem, if so dress with any spent compost or fine peat or peat substetuet the clay is full of nuterants that need releasing by binding the clay togather, this may sound daft but the reason clay is so difacult is because it is so fine,the peat or compost will in time will turn the clay in to granules just like soil only better.

I have heavy clay I scarify and then use a ready mixed top dressing from a garden centre (contains sand) and brush well in. I did mine in September I think its to late now as the grass is very wet and will not look good after.

You should spike roll your lawn with hollow tines, making holes to receive a mixture of top dressing, as Drc says, containing sharp sand, a lawn sweeper without the bag on will brush it in. If you have a large lawn choose a dry day to do this work.