After 3 Plusnet routers and a similar number of visits by Openreach engineers the speed problem appears to be rectified. By installing an old BT router I had.
The Openreach engineer came out and again was baffled by the difference in speed before it goes into the router and the speed coming out. A difference of circa 10Mbps. I mentioned I had the old BT router and he installed it and lo and behold I was back at the speeds of circa 14Mbps!
I just hope now these speeds are at least maintained until FTTP arrives.
Thanks to all who offered assistance.
