"K" wrote after
Nick asked (snip)

Light and better conditions are now available to the lilacs.
If I take some of the weight out of the crowns are they ever likely to do
well or succeed?
I really don't want to cut them down. My wife and I enjoy both the plants
and the blooms but there is also some sentimentality on my part.

If they were mine, I would cut back the almost horizontal 'trunk' by about
half, leaving any branches that are growing near the 'base'. That would
encourage more new shoots from the 'base', which, now the leylandii are
gone, would tend to grow vertically. They would start flowering in about 3
or 4 years. Once the new growth is really established (2-3 years time),
you can cut the horizontal stuff back more and more.

Even though lilac is tough, I don't like cutting back anything too
severely because there's always a slight chance it won't recover. So I'd
look to be pruning and re-shaping over about 3 years (with other shrubs,
an accepted technique is to cut about a third of the branches right down
to the base each year).

If you do do the job gently over a few years, you can always keep a
straggly branch or two to make sure you have some flowers each year.

Don't overdo things, and let the plants teach you how to go about it.

This heavy pruning can be done over the winter (not when frosty) while the
tree is dormant.

After-flowering pruning in the spring is light tidying up of a tree that
is already in good shape. Doing it in spring allows the whole summer for
regrowth and formation of flower buds - you won't get any flowers the
following year on branches that have only had the autumn/winter/spring to
grow in.

A few years ago, whilst helping some new neighbours to reclaim their garden,
we found a large Lilac tree that had fallen right over with most of the
large trunk rotten (subsequently found it was a home for Stag Beatle
larvae). I pruned it with a chain saw right back to the original trunk/root
system leaving just a few new growths that had sprouted on that part of the
plant. Whilst it didn't flower for a couple of years it's now a very healthy
large multi stemmed flowering shrub.

When back to how you want it keep it there with hard pruning after
flowering. Prune at another time and you will lose the flowers for next
season.

--
Regards
Bob Hobden