I've not posted this until today, because my blog's been poorly, but the BT engineer came round last week and upgraded me to FTTC.
I'm now getting about 38Mbps downstream and about 8Mbps upstream.
I'm sticking with Andrews & Arnold, rather than changing to BT Infinity, which slightly confused the BT OpenReach engineer.
The deal is that BT will supply a VDSL modem, and the consumer (me) needs to provide a router that speaks PPPoE. In the usual case, this is the BT Infinity box. In my case, it's a Linksys WRT54GL, running Tomato. I'm pleased to report that this works fine.
Once you raise the QoS bandwidth limits, that is...