The burden of proof is on you

In the event of a Self Assessment investigation being opened the burden of proof is on you. In short, you are guilty unless you can prove otherwise.

So let’s say that four years ago your mother gave you a cheque for £100 on your birthday and you put that into a personal account. HMRC will search out that account and ask you to prove where that cheque came from - they will most likely want to see a statement from your mother’s bank account confirming the cash was a gift.

And what if you can’t do this? What if you can’t remember where the cheque came from? What if your mother has not kept her statements? Well, the probability will be that HMRC will consider that amount to be undeclared income, as a consequence you will not only be taxed on it but you will also have to pay interest and penalties too.