The requirement, as per the immigration rules (https://www.gov.uk/guidance/immigration ... attributes
(i) A letter from each financial institution holding the funds, to confirm the amount of money available. Each letter must:
(1) be an original document and not a copy,
(2) be on the institution’s headed paper,
(3) have been issued by an authorised official of that institution,
(4) have been produced within the 31 days immediately before the date of application,
(5) confirm that the institution is regulated by the appropriate body,
(6) state the applicant’s name, and his team partner’s name where relevant,
(7) show the account number and,
(8) state the date of the document,
(9) confirm the minimum balance available from the applicant’s own funds (if applicable) that has been held in that institution during a consecutive 90-day period of time, ending on the date of the letter,
I am assuming points 1-8 are already in the letter, but you are having a problem with point 9.
If the bank is not willing to state that the balance has been xxxx between start date and end date in the letter, it should be fine to attach the 90 day statement to the letter, but have the bank make the statements part of the letter. Say, by including the words "statements for month xx to xx attached". Even if that is a line at the bottom of the letter that says "encl. three month bank statement" or something similar, it should meet the requirement.
It can also be a separate letter. Some banks are willing to give a letter saying the balance between so and so date was this and that may be one of their standard letters.
My comments are in no way meant to be advisory. I have no professional knowledge of immigration. These are based on my own experience, convictions and personal interpretation of publicly available information.