For a week or so now my Infinity Option 2 has been down to about 1Mb/s download, with about 6Mb/s upload.
Not a big downloader (though if I was I wouldn't know, since the usage checker doesn't work for me) so called the technical support line of India's phone company, which is apparently where BT is based nowadays.
The chap I spoke to (and spoke is a loose term, since he just read from a script) told me that a fault was found on the line and that it would be dealt with the next day. According to BT's online fault tracker, it lies somewhere with their retail servers.
Apart from that, no one has any idea what the problem is, and it turns out that since the broadband is still connected (albiet at snail's pace), they're more or less content to leave it. No refunds for late fixes are possible, as apparently this only applies for times when the broadband is taken out completely.