I've now been in contact with the uploader on Underground Gamer asking him for information on the source of the magazine archive. He said it came from www.zzap64.co.uk and on looking at the site it does seem to be the source.

That site actually sells DVD archives for many different Commodore magazines, and of interest especially to DC will be the fact that most of the collections are for complete scanned archives of mostly C64 magazines, with a couple of Amiga ones.

The site is selling each collection on DVD for £11.99 which isn't exactly cheap, but when you consider how long it would take to scan the lot in it's not actually all that much. Check out the site to see the other DVD collections on offer at http://www.zzap64.co.uk/zzuperstore.html