Hi Stephen,
Thanks for the reply. Yes, I'm aware of the different modem codes available, I was on 332201_A the recommended one for the UK.
Unfortunately, the low throughput came back again with the 12v PSU, so I did try reflashing to 321311_A which is recommended normally for poor lines, mine is a good line but I thought it was worth a try anyway, but it made no difference. When I've tried different modem codes before on other lines, the difference has usually been fairly minor - worth doing (perhaps 10%), but not a massive difference, so I haven't tried any other codes as I don't expect that to be the problem.
I've gone back to the 9v 500ma PSU to ensure I don't kill the Vigor 120 and have contacted Draytek asking them to replace the whole thing. I do think the bad PSU is quite likely the cause - I suspect the reason the problem has come back later with the 12v supply is that the modem seemed to be running hotter (as could be expected) on 12v, so I got the full potential of the line at first, then after a while I think a processor inside the Vigor 120 has throttled to prevent overheating, reducing throughput. A bit of a wild guess, but it's the best I can do.
I'll update the thread once I've got a replacement to confirm if it's fixed the problem.