But am I right in assuming that a couple may enter into a "private" separation? I was reading The Gentleman's Daughter and Amanda Vickery cites a case where the wife was forced to sign a deed of separation that was very unfair to her, but that was all she could do about it because her husband was threathening to have her commited or transported and was practically starving her. Had her brother supported her (he didn't because of economic interests), could she have sued her husband at law? (or in equity?)

Private separations were deeds drawn up by lawyers and signed by both parties, which set out the terms on which husband and wife agreed to live apart. They were quite common means of "divorce" (without remarriage, of course) among the gentry. The problem was that it proved, on the whole, that the terms of separation did not have legal validity if either party chose to challenge them subsequently on the grounds of rights conferred by marriage. Private separation agreements worked fine, but only if both parties abided by them.

Husbands had the duty of supporting wives. A wife could sue for restitution of marriage, including living together, if her husband deserted her. But the way in which a husband chose to "support" his wife was very largely up to him. Only in cases of gross abuse and mistreatment was there likely to be any outside intervention, and even this was uncertain.