If ever there were an object lesson in the need to prove every element of an action it is the judgment of HH Judge McKenna (sitting as a High Court judge) in  Oldcorn -v- Southern Water Services Ltd [2017] EWHC. The claimants succeeded on most issues but were reliant upon expert “modelling” of events to prove that the defendant’s breach caused a flood of their home. It highlights the danger of being wholly reliant upon expert evidence in proving an essential element of a case.


The claimants’ home was flooded due to surface water flooding. They alleged that the flood was caused by the defendant water authority negligently inserting a tideflex valve into one of its own pipes .This, the claimants’ asserted, significantly reduced the reduce flow through the pipe, led to water backing up behind the tideflex valve and caused the flooding of their property.


The claimants’ succeeded in:-

  • Establishing a duty of care in nuisance and negligence.
  • Establishing that the defendant had been negligent in failing to carry out an assessment of the effect of the installation of the tideflex.


To succeed the claimants had to establish that there was a causal link between the tideflex valve and the flooding of their property.    To prove their case on causation the claimants were reliant upon modelling, the claimants’ expert was Mr Allitt. It transpired that modelling is not a precise science.


  1. What was said on behalf of the Claimants is that Mr Allitt, a very experienced modeller, was objective and independent and prepared to make concessions where appropriate, for example in connection with the extent of the impermeable area, and was measured in his answers. The Defendants by contrast, are very critical of Mr Allitt, describing him in their Reply to the Claimants’ Closing Note as an appalling witness whose answers were almost always discursive and argumentative and seemingly designed to prevent any constructive consideration or assessment of his opinions. Whilst I would not be quite so critical of Mr Allitt, there is no getting away from the fact that many of his answers were indeed discursive and argumentative. That said, I accept that he was doing his best to assist the Court in what is plainly a difficult issue. That is the nub of the problem. The fact of the matter is that both experts were having to deal with an exceptional event and as a result were having to deal with areas such as permeable and impermeable which were going to behave differently than they would have done in a less severe storm and were doing their best in their different ways to model flooding so that, for example, whilst Mr Allitt was critical of the Defendants’ adoption of a dummy area of the size actually adopted, he did accept that modellers sometimes do adopt a dummy area as a necessary requirement. Mr Allitt had to concede that his adoption of 16.67 hectares wasn’t correct and he fairly said that he couldn’t say what the correct figure was, albeit that he considered that the overall effect would not have been significant, without quantifying that effect. Both experts adopted different figures for the rainfall uplift. Their respective figures were wholly subjective, and in the case of Mr Allitt were based on his belief that the rainfall on the Estate was in effect 13.5% less than that which fell on the nearest gauge to the catchment. When to that is added the absence of any evidence of surcharge of foul water which would have had the effect of reducing the maximum height of the flood waters in the model and the discrepancy in the timing of the flow. The totality of the evidence leads me to conclude that this court cannot be satisfied on the balance of probabilities that but for the installation of the Tideflex, the Property would not have flooded.”


This is one of a number of recent cases where the action has been about insurers attempting to recover their outlay.   The actions have failed on the fairly fundamental point of failing to prove essential elements of the case.

The proving things series