Both sets were / are 200mbps. I'm just going to take them back now, they are in their own sockets with no extension cables, I've changed the Ethernet cables several times, the software says they are connecting to each other at 190~M though I guess that should be divided as they have only 100m sockets on. Totally stumped, the only difference being last time the modem was upstairs, mac downstairs, this time modem downstairs, mac upstairs but using exactly the same sockets.

The best way to confirm that they are downloading at high speed and then dropping off is to use either Windows Task Manager or tbbMeter. Just have them running and watch the graphs for downloads and PC to PC transfers. You may not be able to get exact figures but if as you suggest there is almost a step in speeds, you will see it happen.

Thanks, I only have one machine and it's a Mac so I can't do one to one tests unfortunately. I've got them exchanged for another pair and will try them later, fingers crossed. What's worse though is they are from PC World who will likely blame my house / wiring / sunspots / dark matter before eliciting any financial recompense from them.