Ok guys, I have found the source of the problem, now I just need a fix
My dad runs a proxy, to block all manner of shite coming through, this means that all my traffic has to go through port 8080.
From what we found if we opened port 80 then the launcher would start, but when it was downloading, it would stop at 99%, then tell me download failed.
EDIT: also leaving the launcher without doing this for 10 mins causes it to run, but won't even start downloading before it gives teh error: Download failed