“…it is clear that special considerations apply to libel actions which are relevant to the exercise of this discretion. In particular, the purpose of a libel action is vindication of a claimant's reputation. A claimant who wishes to achieve this end by swift remedial action will want his action to be heard as soon as possible. Such claims ought therefore to be pursued with vigour, especially in view of the ephemeral nature of most media publications. These considerations have led to the uniquely short limitation period of one year which applies to such claims and explain why the disapplication of the limitation period in libel actions is often described as exceptional.”