Just want to close this thread with a big THANK YOU and WELL DONE to the tech and support guys at plus.net.
Openreach engineers were out and opening the pavement on both Sunday and Monday 8 days ago. The guy on Monday said there was definitely a fault in the cable underground between some connections below the pavement and the cabinet. By Monday evening, the fault was fixed and for 7 days and counting we now get a connection which stays up and is stable and speeds of around 25Mb/s down, 6Mb/s up.
Plus.net also emailed to say that some setting for speed was at 8 rather than 40 and they had changed it.
So to summarise
BT consumer have spent TEN MONTHS telling us repeatedly there is no fault at all.
BT retentions [Truro] have attempted slamming our broadband back to BT and despite promising to confirm cancellation of the order they generated and a complaint [I recorded the call], they did neither. Cost several hours due to wait times on their call lines.
PLUS.NET fixed the dropped connection issue within 3 days of reporting a problem
PLUS.NET then were persistent in getting the speed up to around 25 as promised and achieved that 7 days after reporting a problem.
Again well done PLUS.NET support and thank you.