The WAO is intended to cover the items listed in para 44 of the Schedule of Cost Components which are probably not the type of costs you are considering as head office overhead.
If the office has been accepted as part of the Working Area (not just working area) you may be able to recover some of the costs as Defined Cost but you would have to prove it is only incurred in order to Provide the Works ie but for Providing the Works you would not have incurred the cost. Without knowing the full details I think it would be difficult for you to demonstrate this.
If the office has been accepted as part of the Working Area (not just working area) you may be able to recover some of the costs as Defined Cost but you would have to prove it is only incurred in order to Provide the Works ie but for Providing the Works you would not have incurred the cost. Without knowing the full details I think it would be difficult for you to demonstrate this.