The above issue fixed itself the next day, so either someone read this thread and fixed it or the problem was fixed as is was causing other issues.
I left the Thinkbroadband trace running though and noticed how busy the connection is. See this image:
http://www.thinkbroadband.com/ping/s...8-07-2016.html
This is not down to our traffic (as we are not here 24/7) but must be due to traffic in VM network. As we used the fixed 5 IP option I wonder if this could be due to congestion in networks provided to enable that option (which I think involve us effectively having a VPN into router which redirects the fixed IP traffic to our modem).
That variation in ping times does not seem to cause too much of a slow down but is far from ideal. We also have Zen/Openreach FTTC line too and the variation in ping times is normally only around 20 ms (so lots of black on the graph).
Anyone know if VM are close to releasing the 200 Mb/s service with multiple fixed IPs yet, several of their estimated release dates have passed. Hopefully it does not pass through the same parts of VM's network?!