Clause 63.1 defines the point in time which separates actual and forecast Defined Cost in a CE quotation assessment. Having said this, however, I personally believe that it is the point in time when the quotation assessment is actually made that, in reality, separates the two. This is because a forecast is essentially the prediction of an outcome, in this case Defined Cost. If you know what the actual Defined Cost is, then your forecast should be a very accurate assessment.
A risk allowance relates to a forecast component of an assessment, that is it has a 'significant chance of occurring' which means that it has not actually occurred. Having said this, there may be residual risks which continue after the event, such as disruption, or the profile of a risk may have altered due to the CE, for example subsequent external works now programmed for winter rather than summer. In this instance you would be entitled to include an assessment of these in your quotation, where they are due to the CE.