My suspicions about the devolo kit being on the slow side have turned out to be partly true, but they were really the least of my problems. I took an hour or two to revive an ancient laptop (from when MMX instructions were something to brag about) to test different points around the network, configurations, etc. As old as the laptop is, it's quite capable of doing a few things, and it's reliable.
What I couldn't figure out was why my new PC was only able to move things over the LAN at 0.3Mb/s! It's almost modem type of speeds. Because of the way my flat is laid out it was awkward to isolate the new machine and find out the exact cause. I (literally) dusted off an ancient laptop, and pressd it into service.
After continuously rewiring most of my flat and retesting, it became clear that my new PC, all on it's lonesome, was the cause of the glacial network throughput. After some web searches, auto-scaling on vista was pointed at - no luck (didn't think so, there weren't routers in this path).
More web searches suggested NAM wasn't good. It took awhile longer to discover that NAM is Nvidia's "Network Access Manager", and it's rubbish. I uninstalled it.
Without the devolo kit or NAM in the loop, network throughput jumped up towards the expected speeds (100Mb/s). I've put the devolo kit back into it's original places, and it's not too quick - ~11Mb/s; but my best option right now (and thirty times faster than NAM was forcing me into).
During testing I found that the devolo kit, when installed in sensible places (no power strips, etc.) will get close to it's theoretical speeds. Happy days - if you have power sockets free in the right places. I'll make do.