Resolution (well at least good enough for me)
Well I crawled around underneath our house and updated some of the really old and dodgy looking wiring. The result is an increase from a consistent 9 mbs to a consistent 12 mbs. Judging by the speeds told to me by several neighbours who use the same carrier that's a bit more then what they get, so I'm satisfied.
FWIW, the jump from 9 to 12 came from one thing only - disconnecting a spare outlet in the rumpus room. Disconnect, and the speed jumped. Reconnect, and down it went again. It can stay disconnected as we don't have much need for a landline any more.
cheers, and thanks for the advice.
Arron