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Excellent, thanks for the help both.
a) You are completely correct about the spindle length... silly mistake on my part. Measuring with the preload ring removed gives me a regular length of 110mm.
b) Regarding the spider lockring, I'll try to find one of the normal 3d+ variety and try that. The lock rings seem to be reasonably available from P2M and Velotech, but the only place I can find the tool is on the P2M site for $60!! Does anyone have one that they'd be prepared to lend me (I'm not in London so would need to post it back and forth...).
I really should just give up and buy a Miche Pistard Air, but the sunk cost fallacy has got the better of me.
Right, just dug out my spare Cervelo spider and lockring and an old BSA30 BB. Found a spare 165mm Rotor 3D+ crankset. Happy days. The BSA30 BB fits in the recess in the spider taken up by the weird lockring, so I reckon if you can source a standard Rotor 3D+ lockring like this:
then you should be OK.
Your measurement isn't the same as the one in the photo as you still have the preload adjuster (the bit with the little bolt in it) fitted. And the NDS crank arm isn't supposed to come off.