You don't need any more than 3 to 4 inches of soil to make a lawn but the ground must be well drained, not a hard surface. You need to break up the area in which you want to make your lawn. Bulk bags of top soil are the best buy here in England.

Thanks Db, ......sadly the area under the lawn is very compacted; with more stone than soil. I thought a more practical idea would be to make a 'gravelled area' (using large pieces of gravel) and intersperse it with plants growing through prepared 'holes'. However, she wants a lawn....