I need to customise the 'Recent Content' block to show only 1 content type (i.e pages or articles).
This doesn't seem to be possible with the block settings so I presume that I need to clone and/or edit this block to restrict the content it shows?
A custom View (as a block) would solve your problem.
You can make it replicate the functionality of the 'Recent Content' block but minus the the Content Type that you wish not to include.
Views is the answer, it is the most used module on Drupal sites, but it has a learning curve (well worth studying!). Try the video series at Nodeone.se called Taming the Beast. You will emerge as a guru of Viewdom.
If you want to use Views for creating recents content block, simply create a view, add sorts by newest items filter and select content-types you want to use. You can also add more filters and aditional options.
+You can also click configure from recents block, choose 'content-type' and select content-type you want to limit.