Hi Malcolm,
The 12F675 has comparators, which you correctly turned off with CMCON and VRCON but it also has an A/D and the the pin needs to be set to digital (I think).

Try setting the ANSEL register and set the port to output with the TRISIO register.

Code:
TRISIO.0 = 0   'Set GPIO.0 to output.
ANSEL.0 = 0    'Set GPIO.0 to digital.
I think that is your problem.

/Henrik Olsson.