Immovable property is covered under limitation Act 1963

I purchased a second-hand flat in December 2004 in Chennai measuring built up area 645 Sq.ft from an old lady who had died 10 years ago without any legal heirs. Since the flat was promoted/built in 1977 by TNHB at that time there was no legal concept of Undivided Share (UDS) of Land, no mention about UDS was in Parent document/Sale Deed. So, the Advocate who prepared my sale deed in December 2004 presumed UDS to be 1/3rd of built up area and arrived at UDS as 215 sq.ft. Now, when approached bank for mortgaging my flat, the bankers say that only 1/3rd portion of UDS of land is conveyed to us in the Sale Deed and remaining 2/3rd or 430 sq.ft UDS of land is still held in the name of the Seller. My doubt is 1. Whether the above opinion by the banker is correct. 2. If so, what should I do to rectify this inadvertent error. 3. Now, the Seller is no more expired 10 years ago without any legal heir. 4. Now that flat including many other surrounding flats with other survey numbers were demolished as we signed Joint Development Agreement with a builder to construct new multi storey apartments and new joint patta comprising multiple survey numbers in all old flat owners name obtained. 5.Since, new joint patta obtained which does not contained our sellers name, do I need to execute a rectification deed. 6. How to go about this.