There are 2 forms you can take
1. Statutory
2. Contractual
There are guidelines for late payments which can be followed as stated above. However should you wish to take the Contractual route and make it part of your standard terms you may wish to put this on your invoices where it can be clearly viewed and not hidden on the back or in the middle of small printed text. In deciding how much percentage to charge you need to establish how much and long it would cost you in time and resources undertaking a debt analysis each month. There is no stipulation as to percentage or a fixed amount however should you chose a percentage you must explain how it is applied. i.e the sum of X% will be charge on invoices exceeding 30days etc.,
I hope this help. Please refer to the link above for insight on how interest is charged. Also note that you cannot combine both methods. You may want to seek advice before deciding on which method to use. Nonetheless you will always have the right to a statutory recourse for chasing late payments under current legislation.
I hope this helps