Apparently, being just shy of 3km from the exchange *really* sucks for ADSL2+. While 2km is generally rated for 17Mb/s out of a theoretical max of 24Mb/s, at 3km you're lucky to get 5Mb/s. We've managed to get 5.5 by getting Be to tweak the settings and reduce the noise margin to 3dB, but that puts us at the mercy of the weather.
Then again, testing (downloading large files from somewhere with bandwidth to spare) shows that on the 5.5Mb/s Be connection I can reliably get 4.5Mb/s, while the 10Mb/s ntl Virgin Media cable link seems to top out at 3.5Mb/s or so. This may be related to the higher upload (>1Mb/s) for Be.
Now all we need to do is convince the router to be a bridge, and then actually hook it up.