1. There is no tax on the monies in your hands. Inheritance tax is payable by the estate of your late mother. Any monies you receive as a beneficiary will already have had tax removed if any was payable. So there is no second taxing of the monies in your hands. So the £57,000 you received already has had any tax paid on it and is not of tax. So you don't have to pay any tax on the monies.
2. Effectively, the revenue ensure that all inheritance tax is paid before it ever reaches a beneficiary's hands. That is why Inheritance tax is payable by the estate and not by the beneficiary.
I feel obliged to inform my local council who have been helping me with part of my rent - should I do so
I meant to add on to my question that I feel obliged to inform my local authority of monies I have inherited as they have been helping me with my rent. What should I do as the loss of this would smack of a penalty on my mother's already taxed legacy to me.
3. It is up to you as to what you want to do. It is only if the rent assistance is means tested that you have to declare it to the council. so check the terms of the rent assistance