Phoned Telewest and they switched my Surfboard SB3100 next day. Unfortunately to a Webstar v1, so it didn't improve. Next day they replaced outside cabling (which was too old anyway) and the day after that, replaced the modem with the Motorola 5101E after some channel hopping with their tech support. Their call out engineers don't seem to be aware of any potential v1/v2 issues with the Webstar. Must hand it to them tho, they're fast and turn up on time, in London anyway. Impressive.
Result is an easy 1.2MB/s from gamefiles.blueyonder on a single download connection and I've also seen a sustained 12Mb/s for a while

. Hope that lasts. However, plug in my Netgear RP114 router and life drops down to about 6Mbps. Not good for a router spec'ed at 100Mb.
So generally, this worked partly because
(1) the outside cabling is spec'ed for broadband and has good readings
(2) the modem isn't an SB3100 or Webstar v1
(3) machine plugged directly into modem, not through an (oldish) router
(4) they did some channel hopping (may not have had any effect)