It is relevant to the potential costs of any action that you may decide to take based on the potential options available to you.
Your problem is that no one has seen the actual codicil signed by your father and two witnesses - and without that then none of the circumstances you describe above will be binding on the Executor.
You can ask whether or not a Codicil was filed and rejected - but from what you have said it is unlikely.
It was for you to file a Caveat of you wished to prevent Probate from being granted without your knowledge - there is no process which means that you have to be notified unless the Caveat is in place,
Your starting point is to write to the solicitors involved (their name should be on the Grant of Probate ) and ask about the Codicil.
I would also suggest that you get a statement form your step mother confirming what she knew about the matter.
If it is clear that there was a valid codicil which has not be forwarded to the Court then that is Fraud and you can report the matter to the police AND apply to the High Court (not the magistrates court) to have her replaced as Executor.
I hope that this is of assistance - please ask if you need further details