try this: in Synaptic and using the Ibiblio mirror, press Reload, then Search for apt-sources-list and install only this upgrade. Next, press Reload again, you will then be on a different mirror with fast download speed from which you can then do the Mark All Upgrades, Apply, etc.
This answer is so obvious, yet so helpful, that it should be in your updating instructions, No.2, in place of "mark all upgrades..."
By the way, in case you may sense any insult, I was brought up by a brilliant scientist who had a favourite expression: "True genius is a flair for the obvious".
I am about to re-install and this is going to save me significant time!