If you have Internet access to your bank:
1. Raise a request(send an email to HSBC ) from within the web portal for this letter. Mentioned all the details you needs in the letter e.g. a/c no, period and to some extent content of the letter too. HSBC can charge you for this request but they do provide it easily.
2. Alternatively, write an application on paper and include the printed template of the letter you need from HSBC and walkin to the branch and submit it. Make is clear that it is for Visa and very urgent,. Somebody from HSBC will contact you for the same and you can talk them thru the requirements of the letter.
I did the same and got the letter from HSBC. I asked them to provide one letter for two different account i.e. Current and Online Saver and mentioned in letter that combined sum has never reach below the reqd amount in past three months. Mke sure you apply well in advance as letter may take some time to reach you.
3. Walkin into the different branch and ge statement of Online Saver account. I live in Crodyon and they were happy to put stamp on it for me. May be because the branch is right in front of Home Office building
Hope this will help..