Lens Retaining Ring Stuck

Hi, I am looking for some advice on how to remove a stubborn lens retaining ring.
I have a Copal 0 shutter mounted to a recessed Linhof board which I need to remove in order to fix the cable release elbow. I have removed both lens elements and opened up / retracted the aperture blades. Using a spanner wrench I have tried to undo (anti-clockwise) the retaining ring but it's not budging. This is the first time I have attempted this, so am unsure of how much pressure to apply. I read of someone putting the spanner wrench in a vice to gain more leverage, but I'd like to clarify that first before having a go. Any advice gratefully accepted, Rob.

Attachment 221125

I'm wondering if removing the lensboard might help. If the holes weren't drilled precisely in the correct positions on the lensboard, tightening the screws would seemingly place tensions on the retaining ring. Loosening them might have the reverse effect.

Thanks Neil. I had another go with it, this time using a little more oomph and it gave away. Cable release elbow realigned and all nicely back together again.