Got it all sorted out now. For the future reference of anybody else in this somewhat undocumented situation, a company named 'Autoleads
' makes the following products:
PC99-X01, which is contrary to the claim on the packaging, suitable for all Lagunas 1s without a display on the dash, and what I needed. It contains suitable harnesses for cars with 10-pin ISO-style connectors (what my car had) and 6-pin mini-ISO style connectors (which were apparently also common.)
Cars without the display, but which have a 8-pin DIN (round) connector need an additional harness, part number PC9-201 to use with the PC99-X01 interface. This is apparently not included with the original kit because the 8-pin DIN connector was rarer (I presume the style of the connector in the car depended on the factory fit stereo.)
I didn't look into the kits for cars with the display in the dash, but the part number is PC99-X12.
In addition to the vehicle interface side of the kit, you need a stereo interface kit, which in my case was PC99-KEN, for Kenwood stereos. It includes harnesses for both the high-end Kenwoods (which have a 4-pin DIN [round] connector) and the lower end models like mine, which have a single wire 'bullet' style connector.
This was a nightmare to work out, but I'm glad I got there in the end.
There's now so much wiring in my dash that I had trouble fitting it all in! What with the external bluetooth interface from the stereo (mounted in the passenger footwell), the 2-wire iPod direct control interface (I drilled a hole in the back of the tray under the stereo and left the connector in there for the 'pod), the steering wheel interface box and it's power supply which is a 'daisychain' connector for both of the normal ISO leads into the stereo, it's like spaghetti junction...