Your lilac is at the stage where renovation pruning is the best option; its done in winter, and the drawback is, you won't have flowers for a couple of years or so. You'll need a good saw to take it all down to about 3-6feet in overall height. The following spring, new growth will emerge and this should be selectively thinned by pruning out some growth to form a neat, well balanced shrub. With luck, the following year you should get some flowers at a height where you can appreciate them more...ongoing, prune after flowering as usual.

The other alternative is to remove it and replant.

Hubby did this to ours after it had flowered and it survived and flowered the following year. He said it had 2 choices!

Yes agree, it’s good advice, bring those flowers down from the sky, don’t be afraid to renovate, like mentioned do this in the winter and in my experience it does usually flower in the second year.

Thank you to all three of you for your good advice. I suppose I knew somewhere in the depths of my mind that it would be a drastic remedy! I agree with your OH Seaburn it will have two choices! It's such a shame to lose t as it's a lovely colour. So it must go on a list of things to be done next winter.