Carefully guarded in the Bank of England's vaults are a small number of mysterious banknotes that will probably never see the light of day.

They are called "giants" and "titans" and they are used to back the value of the everyday notes issued by commercial banks in Scotland and Northern Ireland.

Each giant is worth £1m each, however the titan's value is £100m.

Banknote expert Barnaby Faull from Spink auctioneers tells the BBC's Chris Bowlby why there is so much secrecy surrounding them.

Listen to the full report on Analysis on BBC Radio 4 on Monday 28 January, 20:30 GMT and Sunday, at 21:30 GMT.