I bought a BMW wiring CD on Ebay, and I have now discovered that the missing wire needs to be attached from Pin 15 on the seat plug to the K-Bus; a black plastic block which sits on top right-hand of the fuse panel. The lower right connector bus in the back of this block has about 8 or 9 wires connected, all coloured white & red, with yellow markings. If you drop the fuse box down, you will see that there are one or two spare connection pins. I ran a similar size wire from one of these to Pin 15 on the drivers side seat plug.
To gain full access to the fuse box, you need to remove the glove box and adjacent facia panel, then un-hook the fuse box from its hinge pins.
The wires can be run (in plastic sleeving) through the centre console, then down under the carpets and into the existing seat wiring loom braided cover.
I also ran my live feeds from the fuse panel. Fuses 65 and 70 (30Aamp) are allocated to the passenger and driver's seats. BMW do not fit the fuse connectors , but they do provide a permanent live supply to one side. This can be 'tapped' into using male spade connectors, and in-line blade fuses, if you do not have the correct fuse connector pins.
BMW do not provide the connector pins in the seat plug either, so I had to use two standard male spade connectors for +ve (pin 13) feed and -ve (pin 14) ground, and a scavenged connector for pin 15 from an old PC plug that happened to have the correct pin size.
The memory functions now work correctly, but I need to get the BMW garage to 'turn on' the ability to match the seat positions to the two ignition keys that we have. Alternatively I may invest in BMW CarSoft CD and a matching OBDII interface. Both can be bought on Ebay, CD = £7 , interface = £60. Useful for a whole bunch of other things to.