The cause was where the cable goes into the back of the router the nut had come loose by half a turn. The router doesn't normally move but there are times when I run a cable directly into the router to isolate from my main LAN. I have the plastic spanner from when it was installed and discovered I could tighten it up. Once tightened up I performed another speedtest and it passed with flying colours. This is the first time I have encountered that problem in over 22 years with VM, so never thought about checking physically as it looked okay. I assumed the fault was with the router itself, so from one point of view I was partially right.