The tomb of an anti-slavery campaigner who wrote the hymn Amazing Grace is among monuments given new protection because of their slave trade links.

John Newton's tomb at Olney, near Milton Keynes, is one of four to be newly awarded Grade II listing.

Another 21 are having their listing changed or upgraded to mark the International day for the Remembrance of the Slave Trade and its Abolition.

The government said they should be preserved for future generations.