I've now had the seat out of the car to have a proper look. Usual stuff first - 10mm nut to slacken the battery negative, then remove (that is, if you don't want your airbag light on afterwards!!). Leave 15min. To remove the seat there are 4 torx bolts that hold the seat rails to the floor (but a 6mm hex will work). Then you need to prise of the cover cap that allows access to the seatbelt end, to revove it from the seat. You need an 18mm socket for that, after you've removed the one philips screw from the upper seatbelt cover on the side of the seat. The whole seat will now be free. You could simply tip it back to leave it hanging on 3 wires (one airbag cable, housed in a plastic box towards the front of the seat, one lumbar adjust cable and one seat heater cable [black and red]), or disconnect the wires and remove the seat altogether to work on it in the house (a lot more comfy).
To remove the plastic side cowl you need to prise off the cover over the height adjust lever, and remove the single torx screw under there. The height adjust handle will become loose. Pry out the centre of the backrest adjust - there are two spring clips that grip a square shaft - the whole knob just prys off (use a broad screwdriver underneath). It should now ne held on 2 wires - one red LED wire that goes off to the seat heater gubbins, and the lumbar adjust wire, ending in a multiplug. Disconnect both, and the plastic cowl will come away from the seat.
You can now see the (metal) underside of the seat, with the wires going in through the holes cut in it. You can also see the seat heater switch. What appears to be the case is that the red wire goes off to the switch, then back to the heating wire bundle. The black goes direct, and the LED is supplied by the switched live. There seems to be two seat heater pads, each connected with a yellow wire and a black wire. THE PADS THEMSELVES DO NOT APPEAR TO BE ACCESSIBLE. I have removed the anchoring clip at the rear of the seat cushion and peeled back the leather, along with the side anchoring strip, and it looks like the wires go inside the injection moulded foam seat base - there is no getting at them. The leather itself is stitched into the foam, and anchored by impregnated metal wires in the seat pad. It can't be dismantled (or at least I don't see how. You'll see the routed airbag wires and the seat back heating wires while you're looking under the foam. As they say in the Haynes manual - refitting is the reverse of removal!!
So I'm stuck!
It looks like I'll need a new seat if I want to get it going again, unless by some miracle the problem is a dodgy contact further down the wire on the car side, preventing enough current getting to the seat (but reading an OK Voltage)... my only hope
Hope this helps anyone pondering investigating the seat heater wiring!!