Given the behaviour of the multiple thread test things are not looking particularly good.
A recurring issue with Virgin has been that some routes out of the ISP sometimes run hot (i.e. congesting) so this means some sources/files will be good and others bad.
The 1GB download since it is a large single file may behave better than the test which if the 20MB file is being stalled when requesting subsequent copies can cause odd speeds.
There is no reason why with a low latency service that subsequent file requests should be slow, but this may explain the behaviour, i.e. once the stream for the large file is open and running all is well. Also browser downloads actually automatically multi threading.
On the DSL reports 1/1 do you know where the file was that was used?