It is well known that a court can permit a party to amend its case to plead a new cause of action even though, had it been starting such a claim in freestanding proceedings, it may have been statute-barred. CPR 17.2 provides that the court “may allow an amendment whose effect will be to add or substitute a new claim, but only if the new claim arises out of the same facts or substantially the same facts as a claim in respect of which the party applying for permission has already claimed a remedy in the proceedings”.
What if the parties cannot agree as to whether the limitation period for the new claim has expired or not? Such a situation can arise where there is a dispute as to a claimant’s date of knowledge. In Chandra v Brooke North  TCLR 1 Jackson LJ said (at paragraphs 65-67) that there are essentially two options. The court can treat it as a “conventional amendment application”. It will not descend into factual issues seriously in dispute, but rather will consider whether the defendant has a “reasonably arguable case on limitation”. If the court refuses permission to amend the claimant can issue fresh proceedings in respect of the new claim; the defendant can plead its limitation defence and the limitation issue can be determined at trial (often as a preliminary issue).
On whom does the burden lie to prove or disprove the limitation defence? That was issue for the Court of Appeal in Mercer v Ballinger  EWCA Civ 996. The Master of the Rolls decided that the burden was on the Claimant to prove that the Defendant did not have a reasonably arguable limitation defence. Thus he held (at paragraph 27): “The claimant is after all in effect inviting the court to make a summary determination that the defence of limitation is unavailable. If the availability of the defence of limitation depends upon the resolution of 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.”