Hi Tom Riley,
Marketo has a feature called Snippets that is basically what you are looking for. They are re-usable content blocks that can appear in assets like emails and landing pages.
They can even be made dynamic to show different content to different segments.
Another feature you should look at are program tokens, which can allow you to create variables inside your assets that can be updated quickly at the program or folder level. This makes program deployment very fast and easy.
Thanks very much for your answer - will take a look at Snippets and see how I get on.