Regarding the previous answers about Gilts and Bonds - these instruments rarely remain with the original purchaser.
There are various other trades related to them which take place in the Stock Market whereby they are borrowed by or lent out to other parties who agree to pay or receive a set sum of money at staged intervals during the term of the bond loan.
These third parties are gambling that the money they will receive/ pay from the secondary agreement is more / less than what they receive from the original bond issuer or government.
Sometimes these bonds are lent out three four or even more times again in a chain so you have a huge trail of people all gambling and borrowing and lending on the basis of what someone agreed to pay in interest some time ago. Sometimes the banks even "borrow" their original bonds back for a while if they believe they will get a better interest rate from someone else in the chain.
Therefore if the original issuer (usually a government) defaults it all falls down like dominoes. This will then affect other banks and large companies around the world and the knock on effect could be dramatic.
This is why other countries are desperately trying to keep Greece, Ireland, etc... from defaulting on their payments.
It used to be my job to unravel the trail of government bond payments and receipts for one particular large bank back in the 1990's. Greece used to do huge regular trade though not for massive amounts of money. UK and USA were less prolific but offered a better interest rate for investors
Mind you, when I say not massive amounts of money, where I worked £50m ws considered small and the management didn't get het up much about figures under £100m.
It's not only a different world, it's a complete new universe.