Infomation from HSBC this morning.
Yes you do need to take evidence to your bank, but several criteria,
If your bank has copy of your residency, you probably will not need to do anything,
If your bank does not have copy, then residency and turkish utility bill needs to be taken in. (HSBC will have sent you a SMS or e-mail informing you).
If on tourist visa you need to take in a
UK utility bill or a valid
UK driving license with your UK address on. (because officially you do not live in Turkey)
This is for all turkish accounts, not just savings accounts and as far as the bank is aware there is no time limit at present.
Baz