I was going to say Land registry, but you have done this. How about the council tax rolls?
Sadly even though she is seperated, she was not divorced so she is liable for his estate as she was still his wife at the time of death.
If you app;y to the court for Letter of Administration, and your brothers and sisters would have to agree to this and let the court know, you could be appointed administrator of his estate. Drawback to this, if there are debts on the state you all would become liable for them.
Looks like the step mother owns the property if your father took a pay off and this has not been registered with the Land Registry. it could have been between both of them with no solicitor to handle the transaction, so if he signed the deeds over to her, I don't think there is much you can do, without involving solicitors and exptra expense..
I know this sounds harsh, but sometimes it is best to leave sleeping dogs lie and let it all come back and bite your step mother on the backside.