I'm using the Zen-supplied TG582n router, whichis supposed to be fully capable of dealing with the speeds.
The measurements I reported come from three speed-testers: BT, speedtest.net, and thinkbroadband. Although they vary a little, and of course from time to time, they all come in regularly within the same range. On Zen's advice I checked linking direct to the BT modem, which makes no difference. (I also checked using another machine, to rule out problems with my PC itself.)
The results from testers just before posting this are:
BT 47.06 down, 7.09 up
thinkbroadband (Flash) 38.85 down, 9.09 up
thinkbroadband (Java) 40.8 down, 7 up
speedtest.net 45.41 down, 9.10 up