“(1) On an assessment of the costs of a party, the court may have regard to any estimate previously filed by that party, or by any other party in the same proceedings. Such an estimate may be taken into account as a factor among others, when assessing the reasonableness and proportionality of any costs claimed. (2) In particular, where – (a) there is a difference of 20% or more between the base costs claimed by a receiving party and the costs shown in an estimate of costs filed by that party; and (b) it appears to the court that – (i) the receiving party has not provided a satisfactory explanation for that difference; or (ii) the paying party reasonably relied on the estimate of costs; the court may regard the difference between the costs claimed and the costs shown in the estimate as evidence that the costs claimed are unreasonable or disproportionate.” (i) the receiving party has not provided a satisfactory explanation for that difference; or (ii) the paying party reasonably relied on the estimate of costs; the court may regard the difference between the costs claimed and the costs shown in the estimate as evidence that the costs claimed are unreasonable or disproportionate.”
"what in all the circumstances is it reasonable for the client [or paying party] to be expected to pay?"
“(20) The only question, therefore, is whether there are circumstances under which a receiving party, who has certified his maximum costs recoverable up to a certain point under the indemnity principle, can subsequently be allowed to re-open that statement of costs. Perhaps the most obvious possibility is where a party, before summary assessment takes place, indicates to his opponent and to the court that the statement contains errors. In my view, in such circumstances, the parties should make it clear at the earliest opportunity that there has been an error. If, as in this case, the claim for one reason or another does not proceed to summary assessment, then, if the receiving party subsequently seeks to go behind his certification, he must then explain the basis for his assertion. Then it is a matter for the Costs Judge to determine.”