Oh dear! Do I assume from that reply that you're not a landlord?
Also, there was no mention of damage by pets, simply that there is a cat. Damage, if any, by the cat would be assessed at the end of the tenancy. Ground 12 of The Housing Act, where the tenant has broken one or more of his obligations under the tenancy agreement, which I assume you mean you'd try and evict under, is discretionary not mandatory and, without any other Grounds or a Section 8, then quite honestly you'd be wasting your time and money. But, of course, you must know this, your husband is a housing officer.
There are plenty of things that are written in ASTs that are simply not enforceable, or not worth the hassle of enforcing. The tenant keeping a cat is an example of one of them. Obviously, if the tenant was keeping many cats and the property was in disrepair and/or upsetting neighbours, you would have more chance of possession on this Ground.
Thanks for the advice of talking to my local council but, no offence meant, based on what you've said I'll pass on that one thanks.
To the OP, I suggest you try posting this question on the Landlordzone Forums (http://www.landlordzone.co.uk/forums/forumdi
splay.php?f=3)
where you'll get legal opinion not argument.