You can save monthly but each bond is a fixed term deposit
Eg
Buy a 10 year bond in February 2013. Interest paid in February 2014, 2015 etc. Matures in February 2023
Buy another 10 year bond in March 2013. Interest paid in March 2014, 2015 etc. Matures in March 2023
You can buy bonds "manually" when you like or can setup an account that will automatically purchase a bond when the amount of money in the account goes over a certain threshold. You might save regularly into this account but each time a bond is purchased, the money is going into a fixed term deposit.