I reckon I can now answer my own question – it’s not worth risking using the corrector as the instructions (which I’ve just found again) state that for “low power visual use”, the sharpest possible images across the “majority” of the field is obtained by first focusing on the central stars and then turning the focus knob approx. 1/12 of a turn CCW. So in effect nothing would be 100% in focus across the field. This might be fine and not really noticeable for general low power viewing but I wouldn’t be happy making variable star estimates using it in this way.
Cheers,
Mick