I don't think I would write a letter. It's better to go in your bank and talk to a teller about it. They will get things set up the way you want them. They might not get enough information from just a letter.
It should go without saying that if you’re planning to open a joint account with someone - you trust them enough to share financial matters!
You can write a letter (see below) but if I was you I’d make an appointment with your Bank Manager and get it done in person - the bank will almost certainly want two signatures on any forms. It takes about 20-30 minutes.
But.....if for some reason you can’t get to the bank - here is a letter to the bank that you could adapt.
Your Savings Acount Number
Bank Manager’s Name
Bank Manager’s Address
Dear Sir/Madam (or name if known)
With reference to the above savings account (number here), I am writing to request that from (date/month/year) this account be changed to a joint account with Name of Person whose details are as follows:-
Name of Prospective Joint Account Holder
Date of Birth
His/Her current account details - Bank, Sort Code, Account Number
I hope these details will suffice, if not, and if you need further information please contact me at my address at the top of this page.
I still think you'll be called in to the bank - but Good Luck!