The definition of stupidity is trying the same thing twice and expecting a different result right? In this case I was running it a second time to see if it failed at exactly the same point. If that happened then it was most likely a corrupt source/repo on the server. If the values were different (which they were in this case) then it indicated that it could have something to do with the network.
By checking the "Do not verify host certificate" option in TortoiseHG's close function to I managed to complete the download successfully.