Probate has to be awarded before any account is touched; once granted by the courts it gives the executor the legal right to collect and in-gather all the assets of the deceased's estate from their various sources, the executor can and should still apply for probate whilst the account is disputed by the FOS.
Dependent on the dispute however, the executor may have to wait for the FOS enquiry outcome, before removing the funds from the disputed account, before finally closing it down.