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a system where it's your responsibility to tell them
That's largely how the tax system works, outside PAYE as they can get the employers to do the work for that. eg If you work via a limited company your main income may well be from dividends. The only way they can find out about that is when you tell them, either with your company director or your personal hat on.
Could be open to abuse
Risky to abuse it - if they do find out it won't go well.
And, as my experience showed me, they kind of have an idea what they are expecting you to send them, and they will get in touch if it doesn't tally.
Thanks. Yes, I spoke to HMRC today and they said HMRC should get in contact at the end of the year to send me a bill for tax on savings interest (that they've been notified about). I've never received anything in previous years.
Seems surprising having a system where it's your responsibility to tell them. Could be open to abuse.