I found a guy in the ISP who
1) Seems to understand the problem
2) Knows what a splitter plate/I-Plate is (they call it an SSFP)
3) Has English as his native tongue.
A phone line test indicates a 'possible' line problem. We are monitoring overnight with the phones unplugged, and a filter on the router line into the MS - and he has promised to schedule a call tomorrow!
Looking at eliminating the router as a problem since the phone wiring seems to be 'innocent' and it might just be that I have a faulty 'SSFP'.
Bizarrely a test into the master socket (which should not be necessary with the 'SSFP') whacked my speed up from 3.8 to 4.9 and while giving me far less errors, they were still too high.