After a bit more research it seems there are two ways to do this…
create new static mesh (done by right clicking the selected elements in the scene browser) or Merge Actors (which is in the developer tools). Not sure which is the better option but people seem to be favouring the second one.
As I understand it Merge Actors will use one material where it can so if 5 mesh objects have the same texture it will make one call instead of 5. If they are using different matts then these are still kept as separate elements.
Still not sure if this improves performance on the mesh side of things though.