Jtdcftul

I just withdrew £2800 from the bank. Looking at my online banking, there is only a withdrawal of £2000 listed. The form I signed said the amount I asked for, and the teller weighed the cash out on the scales to show the true amount, so my best guess is that she typoed it when entering it into the system.

Presumably this will be noticed at some point since the amount in the system is different to the form, but what are my obligations in the meantime? If I put the "missing" money in some sort of high yield account would I be able to keep any interest accrued? Should I inform the bank or let it slide until it is noticed? In the unlikely event that it's never flagged up, would not mentioning it be theft?

Technically, you have no obligations.

You asked £2800 to your bank, satisfied your bank requirements (ID, having the funds, whatever) and you were handed £2800. You never lied or misrepresented anything and always acted in good faith. You are not required to connect to your online banking to double check the teller's work.

Now, that does not mean that you have won £800. The bank will notice the mistake and will put quite some effort in solving it, not only because of the money but because they want to make clear that the money is always under control (otherwise some employees may begin having bad ideas).

A £800 mistake is unlikely to go unnoticed in a small transaction, and there are probably very few transactions each day comparable to yours, so by the time the bank closes its doors they will probably know what did happen.

They can correct the movement by themselves (either by modifying it or by adding a new withdrawal); perhaps they will inform you (I am betting they will say nothing, because it will not affect you and maybe you will not notice it and nobody likes recognizing mistakes).

In any case, do not try to outsmart a bank. I know of some case (in Spain) when the teller mistakenly wrote an absurdly high amount for a deposit and, when it was corrected, the customer tried to use the receipts to claim that he had indeed made such a high deposit. The bank sued the customer for fraud. Banks do have lawyers, and banks do not have sense of humour. For example, do not try to withdraw all of your money before the bank discovers the mistake, and if the bank calss you and asks about how much money you did withdraw, answer sincerely.

Of course, apart from the obligations, you could always call the bank office and tell them what did happen, as a nicety to them and to save them some extra work and let them know that the issue will be solved in a civilized way.

Do you want a legal opinion or a moral one?

Morally

You should notify them as soon as possible or else some poor employee is gonna have a really, really bad day/week/month/year.

Legally

You don't have to do anything until they question you about the transaction. However, once it comes to light then you had better be ready to either have an additional £800 withdrawn from your account or give them the £800 in cash. They may even freeze your account until the investigation is over.

Let's have a go at answering the three questions posed in a factual manner.

If I put the "missing" money in some sort of high yield account would I be able to keep any interest accrued?

Yes - no problem with that.

Why?

Cash, indeed fiat money, is fungible - and I know that because I read fancy books on monetary theory!

There's no difference between "that" 800 and any other.

Should I inform the bank or let it slide until it is noticed?

I don't see that you have any obligation whatsoever to inform them, unless in your agreements with the bank, there's a clause saying that you should inform them in case of errors.

If there is no such clause in your contracts with the bank, then you simply have no obligation.

In the unlikely event that it's never flagged up, would not mentioning it be theft?

That is a very difficult legal question. (1) Jurisdiction? (2) You'd have to ask some real experts .. law.SE.

As other answers mentioned, the bank will eventually find out and likely just withdraw the £800 from your accounts without notice.

Therefore, I would suggest that you make sure that you have >£800 balance in your account until this happens to avoid being charged any overage fees.