Tax on savings interest but can access records to calculate totals

My only income is from interest on savings bonds. I had a number of 1 year bonds, some paying monthly and some paying yearly interest. I am close to my tax free limit but can't calculate the exact amount as some of the older bonds that finished early in the tax year, I no longer have access to the records for so I can only estimate. I believe that the banks report this info to HMRC so my question is will I be notified of any tax due automatically or how do I request this? If I fill in a SA and it's incorrect will I be either fined if I've underdeclared or refunded if I've over declared? Thanks David

Hi David Mackenzie,
If you file a Self-Assessment return, contact the providers for untaxed interest details to declare. Check if you need to send a Self Assessment tax return.
You can do this via your Personal Tax Account (PTA) via HMRC. Please see guidance at: Personal tax account: sign in or set up. 
If you don't file a Self-Assessment return then wait for HMRC to review your record. If there is underpaid tax, you'll receive a calculation by post.
Guidance is available at Tax on savings interest: How much tax you pay.
Thank you.