Well I have resumed testing with Jdast and extended the download test time to 15 seconds with multiple threads. So far the symptoms are the same, first 5 seconds full speed then starts reducing and getting a bit wobbly for the remaining 10 seconds giving an average download of 15 mbits. If I leave Jdast at its default settings of 5 second test I get 16.4 mbit average.
Have ticket open and waiting for NOC to get back to Vivaciti to try and explain.