Thanks for the replies.
I have to admit, I'd prefer another woman not to actually nurse my child on a regular basis, but I think that's to do with the relationship with my daughter I feel BF gives me. Someone else nursing my baby on an occasional basis wouldn't bother me at all. My Mum BF my cousin once as she was babysitting her, BF my brother and my Aunt was late back to collect my cousin. My Mum couldn't get hold of my Aunt, no mobile phones there, so just fed my cousin as she was inconsolable. My Aunt was absolutely fine about it.
I'd happily BF/wety nurse another woman's baby. In fact, I often feel the urge to if I hear a baby crying, sort of like instinct. I was also suprised recently when my 7 year old (he has Downs Syndrome, so not as developed/can't articulate him as well as most 7 year olds) was tired and crying when we were out, it triggered my let down and I did think 'wouldn't it be great to just nurse him and calm him down...
I would use donor milk if I couldn't BF myself and it was available. Formula would be an absolute last resort for me, not because I think there's anything wrong with the product itself, I mainly FF my older children, but now I know what underhand tactics formula companies employ and that babies actually die because of them, I don't want to give them my money.