All Facelifts have one sensor, fitted to the front nearside, and use 296mm front discs.
I believe all Pre-Facelifts have two sensors, one each side on the front.
Some base model early 4 pot models use 286mm discs and require different sensors and different pads to those with 296mm discs.
All models that use 296mm front discs of whatever vintage use the same sensor, either one or two.
The sensors are a PITA. They usually fall apart if you remove the pads for any reason. So they can't reliably be re-fitted and historically they were over-priced. Hence standard practice for Carltons, Senators and Omegas, going back years, has been to bridge the wires. Not an issue since you should be inspecting the front brakes regularly and not letting the pads get to minimum thickness.
However I recently saw something that suggested that the MOT test now checks that the sensor(s) is/are in place (and the warning light is not illuminated). Can anybody say if that applies to old Omegas or is it just newer vehicles?