Lilac

[counttotenandbreathe]

Total novice gardener here but am trying to sort out the back garden. We have three lilac trees / bushes that are huge but have been strangled by ivy. We have at last cleared all the ivy off but want to cut the lilacs right back as they cast an enormous shadow over the garden. Question is how severe a cut can it take and when do I cut it?
Any advice greatly received.

You can cut the flowered branches now, after they have flowered. If you cut the non flowering branches, you will not get flowers next year, as these branches have the following years flower buds.
If they are overgrown, just cut them back, they will recover.

Also after the good advice above, cut one third of the plant each year to ensure it maintains it flowers and does not get too large.

Thank you both so much I will get chopping once the beautiful flowers have gone great advice on the non flowering branches too