Lilacs can be pruned after they have flowered. You can cut back as hard as you like, and usually you will miss out on flowers the next year but they'll be back the year after that. Suckers should be pulled off rather than cut off but sometimes you just have to make the cut, which leaves behind a dormant bud to grow again.

Thank you Landgirl.......most helpful.

Just as an aside, when I was a youngster and living at home the next door neighbour had a large lilac in the back garden. When it came into flower the neighbour asked my mum if she would like some of the flowers. Mum said: ''No thanks, it's unlucky to have the flowers indoors.'' I often wondered where mum had got the idea. It wasn't until recently that whilst I was surfing the web I came across a site which explained all. It would seem that when someone passed away, the body would be kept in the front room before being buried. At this time of the year when the lilac came into flower, the flowers would be brought indoors to cover any unwanted odours. So, bringing the lilac flowers indoors was associated with death.