Stuff the GPX into Google Earth and you'll be able to verify that it's good.
If it appears as expected in Garmin Training Centre and Bikely why do you need to try Google Earth to prove that it's correct?
No idea about this one, however I've seen some oddness when uploading GPX tracklogs to some sites (can't remember which now) where the trackpoints contained within were not in time order and the website used them in the order within the file rather than sorting by date first.
Still, that wouldn't cause it to suddenly jump over to the other side of Portsmouth Harbour like the SportTracks one has.
You could try exporting the GPX file from bikely and importing that file into SportTracks, remember that bikely exports will have time/date data stripped out.