[boxhead]

I would be making sure that the mounts are good and solid (vibration willkill the alloy cooler)
But taking a guess here, I would suggest the leak has initiated from somewhere close to one of the AN ends, and this has happened due to not using a second spanner to hold cooler part still (preventing from twisting) while tightening the hose.
I realise you said your builder did it up, but just because he is an "expert" at assembling kit cars, does not mean he may make a mistake.
I am a qualified mechanic, and i will put my hand up for doing some stupid things that I shouldnt have, we all can make a mistake.

Or has it been hit by a rock from the road?
A picture will help in diagnosis.