I'm not sure how else to put this, but hopefully the following explanation will make sense...
You would be eligible to apply for ILR, from upto 28 days before the 5th anniversary from your EC date (13 April 2010) as you first entered the UK within 3 months of EC. I.e. (16 March 2015)
http://www.timeanddate.com/date/dateadd ... &ad=28&aw=
So, if your visa extension gives you LTR until 21 March 2015, you will have a period of 5 days within which you can apply for ILR, so long as you meet all other requirements for ILR.
Note that the rules do not say that you need extant leave to cover the entire 5 year residence period. But do state that you need to apply for ILR before you current LTR expires and anytime from upto 28 day before your 5th anniversary.