Jon,
I don't think (note I say think!) it matters what signal the second lambda gets, as all it does is assess whether or not the cat is working. It'll turn the MIL light on, but I don't believe it'll actually affect performance - fuel trim is calculated from the primary sensor only.
The alternative is to send a dummy signal to the secondary input on the ECU - buy a 50p potentiometer from Maplins, and dial in a voltage of about 0.5v to the ECU.
The wiring is as stated above. You'll know when it's correct, because, as Craig says, your engine will run badly until the ECU sees a true lambda signal - it's most noticeable when the engine coolant is above about 75 celsius. For some reason, running my lambda heaters from the ECU I couldn't get the lambdas to run hot enough, so like on every other car I've ever tinkered with, I've just wired my heater to a switched 12v, and bypassed the ECU.
John.
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