The process I followed is relatively funny really:
Unplug hard drive SATA power lead & boot into Windows. Wait until it has finished loading.
Connect SATA power lead, copy as many files possible (using Robocopy) before hard drive starts persistently clicking loudly and stops responding (in practice was about 2.3GB worth of files).
Remove SATA power lead, wait a little bit until the drive spins down. Then go back to step 2.
That was fun, I can assure you. Handily Robocopy does not recopy files that is already copied sucessfully when you re-run it with the same command, so it was actually very useful.
I also tried formatting the larger partition after I had recovered the files, which of course forces all handles to the partition to be closed. It did about 70GB before it started clicking, which tells me the background things Vista was doing were not helping when I was copying files off the drive. BUT if you have a look around on the internet this drive appears to have issues when running under most other operating systems for some reason (the firmware issues, apparently). Booting a command prompt through the Vista DVD would probably normally be a good option - but I wouldn't have been able to address the >1.1TB part of the drive without loading the new SATA controller driver. It could most likely be done but I just wanted to get my data back ASAP, and I didn't think of this at the time. If my replacement drive goes the same fate I may be trying this method, however.
The most ridiculous thing about this was that I managed to check the S.M.A.R.T. values for the drive whilst I was copying my data from it. And NONE of the values were below their thresholds! It had been through so much clicking at the point, I was expecting something bad, but nope.