Boundary wall dispute

A boundary wall dividing two leasehold plots 17 and 18 in a Co-operative Housing Society. The boundary wall of Plot No 18 is encroaching into Plot No 17 by 0.7 to 0.9 metres. The encroachment has existed for eleven years, so the Law of Adverse Possession which favours an encroachment existing for twelve years, I feel does not apply. What is your opinion? Also, is it better to first send a legal notice to the encroacher before going for a civil suit?