Sorry to hear of your father's death.
From what you are saying, it may be that your mother did not have a PPS number prior to your fathers passing. In the event, presumably you have applied and received a Widow's pension for your mother or she was already on OAP - but was this contributory or non contributory. [The latter is means testes whereas a Widows pension arising from your father's contributions isn't.
Unless the estate is complex and there is income in its own right I would have thought that joint assessment would apply to Date of Death. Thereafter your mother would file a return.
The estate only becomes an issue if there is income arising - if the estate has been dealt with and often there is only the Residence and some financial assets to be sorted - are things complicated?