I don't think its the router. I have put back the old BT Voyager 205 to see what happens - works OK but rather slow.
I think it is something to do with BT increasing the broadband speed. They tell me they monitor the setup for 10 days after the upgrade making adjustments to give the best speed on my connection(?). Anyway it needs a few more days to clarify what is happening before their 10 days are up.