Agency changed contract payment terms

[Just an old porsche fan]

Just a quickie and not sure if this should be in SPL but can an agency legally change payment term even if these are defined and clearly setout in the contract?

If not, can I charge a late payment penalty? Which of course they won't pay.

[anothernameitist]

Its depends on what the whole contract says, its commonly known as small print.

Agencies don't you just love them.