“In determining whether to order any payment and its amount, account needs to be taken of all relevant factors including the likelihood (if it can be assessed) of the claimants being awarded the costs that they seek or a lesser and if so what proportion of them; the difficulty, if any, that may be faced in recovering those costs; the likelihood of a successful appeal; the means of the parties; the imminence of any assessment; any relevant delay and whether the paying party will have any difficulty in recovery in the case of any overpayment.”