I went thru this process last year, in something of a panic, declared all my bank accounts and nothing happened.
But I had always declared income, interest etc, in all the previous years, so my total amount due for tax purposes was correct.
However, as I recall, somebody on SFN reported that knew of somebody who had been fined by their local impots.
I took the precaution, I do not know if it was justified, of employing an accountant partly to make sure I used the right forms and partly as I had a professional to support me if things went pear shaped.
An anecdote. Many years ago I had some income which I could easily have hidden. I discussed this with my accountant and his comment was as follows:
You can probably get away with it
You can certainly afford the fine if you are caught.
But the HMRC will make your life a misery from then on and you have better things to do and can make more money making a success of your business than spending time with HMRC inspectors.
So I thought that was sage advice, which I have always followed.
So find a professional, let them guide you through the process.