Thanks Rob,
You've nudged in the right direction and whether you realise it or not provided hints of the right questions to to be looking up so very much value your help.
That link was very handy, and it led me to this :
The pull-up resistor value must be low enough to provide sufficient current through it to power devices on the bus, but not so low that the open drain slaves are unable to successfully pull the data line down to a logic 0 level.
Slave devices are typically able to pull the bus line down to about 0.4 volts when faced with 4 ma of pull-up current due to their internal resistance of approximately 100 ohms. Any additional resistance (due to wiring or other devices) will limit this ability even more. This defines what is referred to as the base floor for logic 0 states. As you may have recognized from this the theoretical noise immunity has been cut in half. To only 0.4 volts (0.8 -0.4 = 0.4 volts). Typical values for the pull-up resistor are between about 1K ohms to 4.7K ohms. This sets the current range to between 5 ma and 1.06ma
(source : http://www.1wire.org/Files/Articles/1-Wire-Design%20Guide%20v1.0.pdf )
So now with a 1k pull-up resistor installed I have a functional temperature probe running at the far end of the 30m tangle of multicore on the floor at my feet.
Rather than take this at face value (I just jumped to the opposite end of the value range so that came too easy) I think I'll write a sketch to log the output over a hot day to pachube or similar to ensure it doesn't drop out when the wire resistance rises due to heat.
Presuming all goes well, we know the answer to this situation with just 1 sensor is over 30m using Jaycar alarm wire. 
Cheers ! Geoff