Tried the quiet line test... seems fine to me. As far as I can tell.
I've been back in touch with the mods again - I've just been at my neighbours, and he can stream movies with his Apple TV with barely a delay. Me? 4 hours + download time even at standard definition. He's certainly not running BT infinity - we can't get it here. Getting very tired of creaky old BT infrastructure - just can't rely on any sort of performance. Anyway, latest BT Home Hub 3 stats below. Note the low connection time - that's because the Hub crashed and needed a reboot.
Turns out that, once the line was fixed, the major fault is with my home wiring - plugging my hub into the test socket (not the master socket - but the test socket behind the face plate) brings the speed up to 4.5Mbps and BT are resetting my line tonight which should bring it up to around 5Mbps - the best we can do in rural Hertfordshire until BT rolls out Infinity to the shires.
Anyway, thanks to all who helped, in particular the forum mods and to Trisha Downe at BT who's taken charge of me and my line.
Until Trisha, everyone I'd spoken to at BT was telling me that 1.5 - 2Mbps was the best I could hope for, and, I have to say the off-shore call centre people, though polite, had no useful information to hand whatsoever ('have you tried restarting your Hub?'; 'Yes!'; 'Do you want to try again...', etc...)