Update 13/09/2014:  We have applied a change to our servers that disabled the problematic component of the operating system that leads to this symptom.  Slow downloads may still occur, but are less likely to fail unpredictably.

Original report:

We are aware of an issue that affects remote access and large files.  When you attempt to download files larger than a couple of MB in size the file will fail to download or fail part-way through the download.  The cause of this appears to be our broadband provider, based on the testing we have done.

As soon as we have an update from the provider we will update this status page accordingly.