I just put together a Google Toolbar Button that takes you to the tracker page for Royal Mail. Just highlight the tracking number (e.g. DP645035876GB ) and press the new button.
You can install it by clicking this link. You will need to install google toolbar (v4) first - its more good google gear and now includes bookmarks. You can get the latest google toolbar from http://www.google.com/tools/toolbar/T4/index.html
UPDATE: My button has been listed in the google gallery Here's my entry.