I can set narrow margins ok.
but I cant see a way of changing the default for a new document?
If you didn’t define a standard template for text documents, Writer will use an internal template that you cannot edit/modify. You need to create a text document containing the settings and the style definitions you want to use, to save it as a template, and to do as @RGB-es described.
If you saved a template anywhere in your file system you can import it subsequently to your user profile and make it your standard using File > Templates > Manage Templates.
File → Template → Save as template → check the “set as default template” option.
The combo styles/templates is very important on Writer.