gst input of fy 2017-18

credit of F. Y. 2017-18 can claimed upto date of filing September, 2018 return or filing of annual Return Of 2017-18 ,which ever is earlier.
so you can claim credit.

As per interpretation, u can claim such input in annual return of DUE DATE of filing sept return which ever is earlier.. after that it will lapse

absolutely correct. so you have to claim in this September return only.

sep return is filed late so can we claim input after 20 oct in sep rerun

As per my view, u cant.. DUE DATE of filing is 20th oct so if u want to claim input u hve to file by 20th othrwse it will lapse. Late filing of sep return will not fulfill the objective of law. Law clearly states two dates: DUE DATE of sep return or annual return, which ever is earlier. Other views are also appreciated