A good friend tells me the Ez7z worked perfectly and easily with split & zipped video files on his Mac where as the 2nd link gave the similar problems
So if you use a Mac - Please use this: http://www.macupdate.com/info.php/id/19139/ez7z
There are 2 options here for Mac users to unzip & rejoin the video files on this site. Both of which I believe work.
Here is a link to 7-zip for Mac: http://www.macupdate.com/info.php/id/19139/ez7z
another utility here: http://www.xs4all.nl/~loekjehe/Split&Concat/
I hope with these we will solve any issues that Mac users have had. In general I do apologise to those who experience problems with un-zipping these zipped & split files. As yet its unclear as to why for some many problems occur where as for others it works perfectly and as such, if I can't clearly identify the problem there's nothing I can do to fix it! Sometimes during the download process files can corrupt and these videos are big files.
Its quite possible that in the future I will be re-encoding many of the Sigur ros / jonsi videos using the new MKV video format which literally makes files half the size of AVI's... but thats for the future :)
Suggestions for better methods are always welcomed