Hi Isobel,
I would sugest you investigate your soil, if possible dig a hole at the side of your lilac tree, and check th soil for structure dryness and wetness.
Check your leaves to see if there are any bugs hidding in the curled up leaves, normaly looks like a cattapillar.
When you planted the lilac, did you dig a good size hole and plant with a soil based compost?
Just as a pointer, my garden drains very quickly, i`m situated on sandy clay, so even though the ground looks wet, unless we have had constant rain, underneath it is still dry.
In a plants first year i would allways check the plant whatever the weather.
To me it sound like its been dry at some point
Mike Easom