IMHO the coercion defence is really a (possible) mitigating circumstance, which would affect sentencing -- not absolve one of guilt.
If it really is the case that statute has a 'my husband made me do it' clause, it's probably time to get rid of that. Hey, let's have it added as an amendment to the gay marriage bill!
No, apparently it's an anachronism that can get you off entirely.
Any presumption of law that an offence committed by a wife in the presence of her husband is committed under the coercion of the husband is hereby abolished, but on a charge against a wife for any offence other than treason or murder it shall be a good defence to prove that the offence was committed in the presence of, and under the coercion of, the husband.
http://en.wikipedia.org/wiki/Marital_coercion
You're right it should go. A recommendation to that effect was made a mere 36 years ago.