Took the new lenses out for a spin over lunch. Everything fine until I pressed the shutter with the 28mm mounted - nothing happened. Checked I'd actually wound the film on, yes.
Switched to the 50mm and it worked. Back with the 28mm, it didn't.
After a bit of head scratching and comparing with the other two lenses, I twigged that the aperture lever was jammed solid, not budging even a fraction of millimetre. :bang:
So, reluctantly, I was about to compose an "I'd like to return this item" message to the seller but thought I'd have one last go and give it a proper shove. Applied some brute force and ping! the lever suddenly came free (in a good way).
Now moving freely and the lens stops down as Olympus intended.