RESULT!
Despite buying a Zyxel VMG8324-B10A, a VMG8924-B10A turned up. Not fussed, put on johnson's vustom firmware from 30th June and all is well. No longer am I losing traffic to failed packet fragmentation, but I'm getting some more speed. Still syncing at 65Mbps but speedtest is 61.55/18.33, 3-4Mb faster down than before.
If there's a better command to get stats, let me know.
> adsl info
adsl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason: 0
Last initialization procedure status: 0
Max: Upstream rate = 24677 Kbps, Downstream rate = 65636 Kbps
Bearer: 0, Upstream rate = 19999 Kbps, Downstream rate = 65664 Kbps
Bearer: 1, Upstream rate = 0 Kbps, Downstream rate = 0 Kbps