so does anyone have any ideas/experience on a type of software which will do this?
A little ;)
We don't want to re-invent the wheel or all of the content
Exactly. If you are developing for mobile (as in NOT an actual app
) but rather phone/tablet/pc then the research work you will want to dig into is as travelin cat
mentioned, buzzword: "responsive design". You can have all your original content served from a single source which means the URI remains a single resource request as well. You can use media queries to server up the proper display format.
how to test if it's a PC or mobile/tablet/iPhone/etc.?
You don't. New devices are coming on to the scene daily with different viewport sizes, etc. As mentioned earlier, you set up your site with media queries to progressively enhance the user experience based on viewport size. I think a picture is worth a thousand words. Get your Firefox browser up and running, and hopefully you are using the latest version. You won't need the Web Developer Extension by Chris Pederick but it also has some neat features for testing multiple viewports. We're going to visit the open source Initializr project demo page which is a packaging site for HTML5 Boilerplate (awesome resource as well, I'll include a link to that below too).
Firefox has a very cool feature built-in. Once you land on the page below, execute this command from your menu:
Firefox > Web Developer > Responsive Design View (Ctrl + Shift + M) to see the viewports one-by-one (selection box at top).
If you have the Firefox Web Developer Extension by Chris Pederick loaded (http://chrispederick.com/work/web-developer/firefox/) you can view multiple viewports by clicking:
Firefox > Web Developer Extension > Resize > View Responsive Layouts
Play around a bit, read up a bit more. It may be what you were looking for. Resource I mentioned earlier:
HTML5 Boilerplate: [html5boilerplate.com
Note: there is a mobile boilerplate version too, but that version is really just to make your mobile version look and feel like an actual app. If you are confused on which one to use you are not alone as the question has been asked many times. The definitive answer from the project team can be found in a Google Groups discussion (note that there is a fragment hash in this url so you may have to copy/paste to get to the resource): [groups.google.com
And finally I want to say this: there are many different opinions on which way you can take your mobile site. I read your original question and this is merely my offering on the direction you might consider.