Ok, I've answered my own question, basically I extended the UrlFileNameViewController to get the uri string from a UrlPathHelper utility class. You have to first set a url decode switch on the helper before requesting the uri (!) which then automatically decodes the uri through the getRequestUri helper method:
Code:
import org.springframework.web.util.UrlPathHelper;
...
UrlPathHelper pathHelper = new UrlPathHelper();
pathHelper.setUrlDecode(true);
String uri = pathHelper.getRequestUri(myHttpServletRequestObject);
...
You can then carry on using the uri string normally in the UrlFileNameViewController.
Dumbdare