I agree tizi, this is odd. There are two types of Letters of Administration. One is Letters of Administration with will annexed, which is when there is a will but the named executors are dead, unable or unwilling to act. The other is LoA - which like you say is when there is no will.
Is it possible that your mother made a new will and perhaps those executors are unable to act?
Failing this, I should contact the probate registry - ideally you want a copy of the oath that the administrator swore. If they won't assist, I suggest you get a solicitor involved.
You will need to apply to have the Grant set aside (this is easier if the other side consent) - assuming the Will under which you are appointed as executor is valid.