If its a Syringia vulgaris variety, you can carry out renovation in winter when its dormant by stooling. Reduce down to 1 or 2 metres, making sure the limbs you keep are well spaced, to create the new framework. In spring, new growth will emerge from the bare wood and can be selectively pruned to form a neat, well balanced shrub. You will not get any flowers for 2 or 3 years though, and you may find there's a big increase in sucker production, which should be removed.

Thank you Bamboo. It sounds a bit drastic, but I think needs must!

Yea, quite a big job, but its not uncommon for Lilacs to become ungainly and unattractive over time, so this, drastic though it is, should renew it.