Joe, Brumsgrove, TJ: Thanks for the prompt replies.
JoeBones: YGM
TJ: Thanks, but it is well and truly cheddared.
Bit of an extract from my mail to Joe:
"…
Didn't go to great lengths in my post but, basically, I've never seen a crossthread like it.
I can only think that, in my haste to go for my first 200 metre spin on my new build (on Christmas Eve), I only finger tightened my surrogate (want-to-ride-it-now) pedals.
Since then I have gone no further than 3 miles – each time in normal shoes/boots (ie Not cleated) – as I was just popping to the shop to get brakes bled etc.
Today, within two minutes of my first proper cleated ride (on this bike) I noticed that they were floating strangely.
On inspection, and though inserted 90% of the usable thread, it was clearly on the p*ss.
I can only think that, during my civvy-shoed rides I'd managed to unthread the finger tightened pedals slightly and then, under load, managed to jump a thread before tightening them back up while SPD-pedalling.
The female thread appears more or less reamed.
…."