Thanks Everybody. Marches Farmer - I know you're "right" about this (but I would say that, wouldn't I, because you agree with my point of view). The key question is how do you handle the situation when your equal partner in life and smallholding thinks differently. Arguing logic as you have done is perfectly ok, but it is on a different plane from the one they're thinking on, and only makes one (i.e. me) look cold and uncaring.
An update on the situation then: Following Sally's excellent suggestion, Mrs Womble and I have agreed some basic ground rules:
1) Any animal that is dangerous has to go.
The problem here was not that Mrs W disagreed with this, but that because Dinky had never shown her any aggression, she thought I was using a minor transgression as an excuse to get rid of a non-productive animal. Once she witnessed his behaviour first hand, she agreed with me that he has to go. He has been booked into the abbatoir next week, along with our three mastitisy ewes (these are another story, but it's been a hard month!
).
2) Any animal which is suffering, or may pass on ailments to others (e.g. infectious mastitis, CLA, recurrent footrot etc etc) has to go. In case of disagreement, we will ask the vet what they would do if it was their animal (with a smallholder head on, so not necessarily thinking purely commercially), and will then follow that advice.
We still have to decide what to do about other animals, for example non-productive older ewes or non-aggressive Dinkies, but this is at least a start.
Also in return for doing in her wee pals, I have to buy her some replacement ewes at the forthcoming Stirling Z auction. TBH, there are worse concessions to have to make to keep a marriage together!