Something some solicitors forget! You can increase your agreed initial budget.
PRACTICE DIRECTION 3E – COSTS MANAGEMENT(D) 7.6
Each party shall revise its budget in respect of future costs upwards or downwards, if significant developments in the litigation warrant such revisions. Such amended budgets shall be submitted to the other parties for agreement. In default of agreement, the amended budgets shall be submitted to the court, together with a note of (a) the changes made and the reasons for those changes and (b) the objections of any other party.