Wanting to test run code at a slower speed, I meant to change the fuses to internal RC oscillator and I rushed and clicked external low frequency oscillator instead.
This is the one that takes a 32KHz xtal on different pins. Oops. Board layout and connections meant adding a 32k xtal didn't work (no surprise).
Just to let all you know the secret to unlocking it was two-fold:
1) apply an external clock no more than about 40 KHz.
2) Apply it to the xtal2 pin NOT one of the TOSC pins where you would connect the low frequency crystal.
Don't ask me why this worked, but it did, and the low frequency was essential to success (having tried various higher ones first).
... and how I know what the problem was? the first thing I did after read signature succeeded was to read back the fuses to see check what I had done.