I would like to see the dogs trust suing for using the phrase "a rabbit is for life, not just for easter". For a start the phrase "a dog is for life, not just for Christmas" was invented by them 30 years ago (by their own admission) & is therefore outside copywrite law. Even without that I would say that by substituting rabbits/easter for dogs/christmas sufficiently make the phrase substancially & sufficiently different. It would be different if perhaps you were selling chocolate rabbits that you described as "finger licking good!" as that would be a 100% copy of KFC's slogun.