Corker,
Firstly, I do not work for Obihai.
I'm not sure OBI is the best solution for what you require. If voip is not a strong requirement, then maybe a small pbx is what you need. £130 will buy you a small PBX, 2 exchange lines and six extensions (from Maplin). Say ext 1 in house could be a dect base unit and ext 2 in office a plain wired phone. I guess you have 2 spare pairs in your cat5 cable to get the phone to the office.
An OBi110 is an option, but only if you are willing to use a softphone on your office computer as the office phone. Then the phone in the house plugged into the main OBi unit could be a dect base unit. This does have the advantage of giving you the option of having two voip providers available to your house phone and office softphone as well as BT.
There are ways of using two OBi units to do what you want, but the configuring starts to get very complicated for what is really only a simple requirement.