Insert a url segment without creating sections I have a very simple site split into sections. All pages use just one template.

The previous site had sections such as /section/subject.  I had thought I could have all entries in just one section and then alter the URL on each entry to include a 'pretend' section /section/{slug}, but this defaults to /section-subject.

Is there a way to have /settings/subject/ without creating new sections?

Many thanks

Martin
From what I gather from your question you want some sort of sub-categorisation of your entries. 

Such that one entry can have the URL as:
    siteUrl/section/subject-1/entry-slug

while another entry can have a URL as:
    siteUrl/section/subject-2/entry-slug

If that is what you want to achieve then you have to look into Structures instead of Channels

Here is the link: http://buildwithcraft.com/docs/sections-and-entries#sections

You can basically have multiple entries with sub-urls in the same structure section.