“… statistics, or at any rate guidance upon research, are now available in the notes to the Ogden Tables which demonstrate that so far as the level of any “arbitrary” or generally applied level of discount is concerned, a figure of 25% is a gross departure from that appropriate simply in respect of future illness and unemployment. In order to justify a substantially higher discount by reason of any additional future contingencies, there should in my view be tangible reasons relating to the personality or likely future circumstances of the claimant going beyond the purely speculative”