Calculating tax when using a savings platform that doesn't inform HMRC of interest earned.

My mother in law is a widdowed pensioner and isn't registered for self assessment. She has recently started to use the Active Savings savings platform on Hargreaves Lansdown but has been informed that Hargreaves Lansdown don't inform HMRC of the amount of interest earned and nether do the registered banks. They only provide an interest statement. It's likely she will be over the personal allowance and will have to pay tax but normally HRMC get advised and adjust the tax code. Can she report the interest earned without having to complete a full self assessment?

Hi
Provided the total interest received from all sources, is below £10,000, then a tax return is not required to declare the interest. Instead, a letter to
H. M. Revenue and Customs
PAYE
BX9 1AS, declaring the interest as shown on the interest statement is all that is needed.