"Can someone point me in the right direction as to how we go about this please, and if there's any other recommendations / pointers you could suggest, then please do so as this situation is feeling quite daunting, under the circumstances."
It really is not that daunting. Your brother can renounce and someone else can take out a Grant. Personally. Or instructing a solicitor.
I'm a solicitor and I will always recommend that someone , in your / your brother's circumstances, instructs a solicitor to act. I will often work with a family, one of whom is named as executor but needs a lot of help from family to deal with all the issues - including the non legal issues like clearing out a family home. They can still be the executor and for a lot of people this is what they want, to carry out their father's wishes.
There is a temptation to try and deal with Probate personally - because it is "so simple". Sometimes, it's just not and, if people are feeling overwhelmed, best to take proper advices.
mf