My BB was running at a steady 3500 for ages (according to Speedtest), then slowly month by month it started dropping until it got to just under 2700.
I asked my ISP to investigate and after a new request to BT to monitor my line I am now up to 4800
I think that my old Alcatel Speedtouch 330 was proving unsuitable for speeds greater than 2Mb but now that I am running a Netgear G+ modem router greater speeds are possible.