At the time I was the only one connected.
I have since asked my son to test with his wired connection (too far for wifi) and was amazed he still had a perfect connection.
I have since tested my wifi with similar results, all showing good x6, but single stream starting high and dropping off, except for this one which seems most unusual:
I have checked my wifi channel and only one other user is using Chn 6 and I'm at 100% and they are 18% using Vistumbler, so interference should not be an issue.
With the wired access performance I guess it must be a wifi issue.
I have contacted Plusnet, but will check mine and other wifi connections over the weekend.