“… provided the defendant can show a prima facie defence of limitation, the burden must be on the claimant to show that the defence is not in fact reasonably arguable. … If the availability of the defence of limitation depends on factual issues which are seriously in dispute, it cannot be determined summarily but must go to trial. Hence it can only be appropriate at the interlocutory stage to deprive a defendant of a prima facie defence of limitation if the claimant can demonstrate that the defence is not reasonably arguable”