“Where the court orders a party to pay costs subject to detailed assessment, it will order that party to pay a reasonable sum on account of costs, unless there is good reason not to do so”
“22. It is clear that the question, at any rate now, is what is a ‘reasonable sum on account of costs’… 23. What is a reasonable amount will depend on the circumstances, the chief of which is that there will, by definition, have been no detailed assessment and thus an element of uncertainty, the extent of which may differ widely from case to case as to what will be allowed on detailed assessment. Any sum will have to be an estimate. A reasonable sum would often be one that was an estimate of the likely level of recovery subject, as the costs claimants accept, to an appropriate margin to allow for error in the estimation. This can be done by taking the lowest figure in a likely range or making a deduction from a single estimated figure or perhaps from the lowest figure in the range if the range itself is not very broad.”