We have chosen to install approximately one temperature sensor per room. Using a vented spacer place the temperature sensor will sit behind the light switches, and the Cat5 comms cable will run (at least for the vertical descent) alongside the mains lighting cable in trunking. I believe this is OK under the UK building regulations, but I'm sure the buildings inspector will let us know. 

The sensor is simply soldered on the end of the CAT5 cable and left loose inside the spacer plate. In the space between floors the cable will run to a punch-down comms junction box (BT type), then back to the control unit. I have yet to actually test a long cable run, so need to experiment with the slew-rate limited outputs. In the worst case the network might require some segmentation and dual transcievers, but I hope with cable runs less than 30m it should be OK on one.

The control unit is still veroboard, no chance of CE approval for a while yet then :-)