Some good news
I took this picture last year because I was amazed that the lilac in the front garden was showing two types of flowers, lilac and white (I was on my way to the pub hence the darkness!).
I suspected that the root stock was sending up a sucker and I wasn't very happy with the health of the plant.
Then it got a bracket fungus which the RHS confirmed was a destructive type based on the photo I sent them.
Following my instincts, I cut out the branches that had lilac flowers (common lilac is lilac duh! but the white blousier stock is like the one I had in my last garden, a french sounding name.
As I weeded around it, I found suckers coming up and snapped them off at source. I built up the soil a bit thinking that I might need to darken out the root stock.