Your question: Do this flat owners have any legal claim on our amenities which is constructed as indicated above.
Answer: Yes the flat owners have right over the amenities which are constructed above, but only over the amenities available on the 6 blocks and not the 7th block. UDS works in the fashion that, the builder and the owner will both have a certain share in the part of the land. Therefore it is very important to read the agreement which the builder has signed with you and the relevant terms and conditions of the Agreement.
I would advise you the following:
1. The builder cannot create third party rights;
2. I would require to read the UDS agreement, including the terms and conditions;
3. As long as the Agreement is registered, the flat owners are the real owners.
4. You should consult a lawyer to verify your legal documents, so as to avoid problems in the future.