Most of the time it's URLs that are long winded and awkward like this, but there are examples of long winded urls that do work just as well.
Conversely, this simple url doesn't work http://documentlogic.co.uk/pdf and it's an incredibly simple rewrite!
Please help, or at least add this as a known issue to the module documentation.
If you are having problems with the Link Mapping module, maybe you should have a look at my SEORedirect module for Silverstripe 2.4.
The legacy URLs/urls to redirect are added to a CTF on the page the link to - that's how i like it. But should be easy to create an ModelAdmin for the redirects.