could you please tell me how far a Lilac tree should be planted away from property wall

I wouldn't worry. I have had one growing against my wall for over 20 years, They are shallow roooted

As long as the plant has been sited along or on their side of the boundary then there isn't much you can do. Unfortunately one tree does not constitute a hedge therefore it doesn't fall under the Anti Social Behaviour Act 2003 section 8 High hedges and doesn't have to be kept at 2m. If the boundary is yours you are able to maintain it from both your property and their side of the boundary also. If the tree grows over the boundary(and it is your neighbours legal responsibility) on to your property you are well within your rights to prune the tree. You are supposed to return the branches you have taken off, but I wouldn't like this, I would dispose of them yourself.
I wouldn't worry too much as Poaannua said the roots are very shallow, but you must record any damage and inform them immediately. It may be worth mentioning you are a little concerned about large shrubs and trees being planted to close to your property, but be polite and keep the lines of communication open and civil.

Miles and miles in my opinion!! Apart from the two weeks in which it flowers it is boring, with dull leaves, brown dead flowers, and no autumn colour...just more brown!! Two feet should be OK for your wall though.

Hi. Volunteer you have made me think, is she worth it. My Lilac is Madame Lemoine A double flowered white variety. but shall I or shan't I . Her fate is in the balance.