Why it's time to check down the back of the sofa

With the new 12-sided £1 coins set to start appearing in our change next week, Brits are being warned to use up their old coins - or lose them.

Nearly half of us have £1 coins lying about the place, new research from Gocompare.com Money indicates, adding up to a whopping £420 million.

And those that fail to round up this cash and spend it will see it become worthless in October, when the old round £1 coins are withdrawn from circulation.

The least common (and most valuable) £1 coins

"From the end of March to mid-October both the old and new £1 coins will be in circulation at the same time and will be accepted by shops and banks. After 15 October 2017, the old £1 coins will no-longer be legal tender," says Go Compare's Matt Sanders.

"So if you've got a collection of old style £1 coins in a piggy bank, coin jar or if you keep some in the car to pay for parking or to use to release the lock on a supermarket trolley – you'll need to round them up and either spend them or pay them into the bank before the October cut-off date. Otherwise, you could be left out of pocket."

Highly valuable coins: cash in on a Royal Mint mistake

As for where you should be checking, says Go Compare, one in five of us have £1 coins in pots of loose change lying around their home, and 13% say their kids keep £1 coins in their piggy banks.

Seventeen per cent keep £1 coins in their car to use in supermarket trolleys, and 15% of people have some in the glove box to pay for parking. Fourteen per cent reckon they have pound coins down the back of the sofa.

The official deadline for using the old coins is 15 October, although some banks are planning to allow customers to bring them in and exchange them for a little longer.

When will old £5 notes become worthless? What should you do?

Meanwhile, the old paper £5 note is due to go out of circulation even sooner, on 5 May. According to the Bank of England, there are still around£165 million worth of notes out there. However, unlike the new coins, you'll be able to change them with the Bank of England indefinitely; there's information on how to do it here.