From citizens information
Appointing an Administrator
If you don't make a will, an administrator must be appointed. An administrator is also appointed where an executor is not named in the will, dies before the testator or is unwilling or unable to act.
The next of kin may apply for a grant of administration. Priority is given in the following order:
the spouse or civil partner,
child,
parent,
brother or sister,
more distant relative.
If there is doubt about who is entitled to be the administrator, the issue will be decided by the Probate Registrar. Usually, an administrator is required to give an administration bond to the Probate Office - this is a sort of guarantee that you will carry out your duties properly