Hard to say without a lot more information - any sign of infestation or disease on the lilacs (assuming you checked backs of leaves, branches and stems); precisely when did you plant each one, and how long did they last? how did they die, meaning, did all the leaves go brown and shrivelled; have you planted other plants in your garden and are they doing okay? Which variety of lilac? what other plants have you planted? Which area of the UK are you in?

Dying after one to several years is commonly caused by being planted too deep, though you will want to check the exact symptoms, as Bamboo says.

When you say new soil what exactly do you mean, and what is underneath it?