I did a thread recently on the demise of an old lilac tree, and it appeared what helped do it was honey fungus. This is the parents' home and there was an outbreak some decades ago. I understand you get to know about it because of horrible alien-type scraggly roots in the earth, and that it's hard to get rid of.
This thread on Mumsnet helped me out a bit:
www.mumsnet.com/Talk/gardening/969817-Anyone-know-anything-about-honey-fungus?msgid=20149319
Thing is, I'd like to replant a new lilac there in the same place. Other trees grow there easy enough - hawthorns and so on, so it can't be that bad. But to be on the safe side, how about I dug up the area to say five feet deep, got out the earth and replaced it with compost and topsoil? Could that work or would I still have to leave it 10 years as suggested on another thread?
Is there a way of doing a garden 'survey' as with a house where someone can take soil samples and see what I'd need to do, or do I just have to take pot luck?